Prefix all the calls to the the regexec family of functions with `p_`.
This allows us to swap out all the regular expression functions with our
own implementation.  Move the declarations to `posix_regex.h` for
simpler inclusion.

Move to the `git_error` name in the internal API for error-related
functions.

use consistent names for the #include / #define header guard pattern.

In commit a390a8464 (cmake: move defines into "features.h" header,
2017-07-01), we have introduced a new "features.h" header. This file is
being generated by the CMake build system based on how the libgit2 build
has been configured, replacing the preexisting method of simply setting
the defines inside of the CMake build system. This was done to help
splitting up the build instructions into multiple separate
subdirectories.

An overlooked shortcoming of this approach is that some projects making
use of libgit2 build the library with custom build systems, without
making use of CMake. For those users, the introduction of the
"features.h" file makes their life harder as they would have to also
generate this file.

Fix this issue by guarding all inclusions of the generated header file
by the `LIBGIT2_NO_FEATURES_H` define. Like this, other build systems
can skip the feature header and simply define all used features by
specifying `-D` flags for the compiler again.

In our CMakeLists.txt, we have to check multiple functions in order to
determine if we have to use our own or whether we can use the
platform-provided one. For two of these functions, namely `regcomp_l()`
and `futimens`, the defined macro is actually used inside of the header
file "src/unix/posix.h". As such, these macros are not only required by
the library, but also by our test suite, which is makes use of internal
headers.

To prepare for the CMakeLists.txt split, move these two defines inside
of the "features.h" header.

In a future commit, we will split out the build instructions for our
library directory and move them into a subdirectory. One of the benefits
is fixing scoping issues, where e.g. defines do not leak to build
targets where they do not belong to. But unfortunately, this does also
pose the problem of how to propagate some defines which are required by
both the library and the test suite.

One way would be to create another variable keeping track of all added
defines and declare it inside of the parent scope. While this is the
most obvious and simplest way of going ahead, it is kind of unfortunate.
The main reason to not use this is that these defines become implicit
dependencies between the build targets. By simply observing a define
inside of the CMakeLists.txt file, one cannot reason whether this define
is only required by the current target or whether it is required by
different targets, as well.

Another approach would be to use an internal header file keeping track
of all defines shared between targets. While configuring the library, we
will set various variables and let CMake configure the file, adding or
removing defines based on what has been configured. Like this, one can
easily keep track of the current environment by simply inspecting the
header file. Furthermore, these dependencies are becoming clear inside
the CMakeLists.txt, as instead of simply adding a define, we now call
e.g. `SET(GIT_THREADSAFE 1)`.

Having this header file though requires us to make sure it is always
included before any "#ifdef"-preprocessor checks are executed. As we
have already refactored code to always include the "common.h" header
file before any statement inside of a file, this becomes easy: just make
sure "common.h" includes the new "features.h" header file first.

Next to including several files, our "common.h" header also declares
various macros which are then used throughout the project. As such, we
have to make sure to always include this file first in all
implementation files. Otherwise, we might encounter problems or even
silent behavioural differences due to macros or defines not being
defined as they should be. So in fact, our header and implementation
files should make sure to always include "common.h" first.

This commit does so by establishing a common include pattern. Header
files inside of "src" will now always include "common.h" as its first
other file, separated by a newline from all the other includes to make
it stand out as special. There are two cases for the implementation
files. If they do have a matching header file, they will always include
this one first, leading to "common.h" being transitively included as
first file. If they do not have a matching header file, they instead
include "common.h" as first file themselves.

This fixes the outlined problems and will become our standard practice
for header and source files inside of the "src/" from now on.

Introduce a simple counter that `p_fsync` implements.  This is useful
for ensuring that `p_fsync` is called when we expect it to be, for
example when we have enabled an odb backend to perform `fsync`s when
writing objects.

`snprintf` requires a _format_ but does not require _arguments_ to the
format.  eg: `snprintf(buf, 42, "hi")` is perfectly legal.  Expand the
macro to match.

Without this, `p_sprintf(buf, 42, "hi")` errors with:

```
error: expected expression
                p_snprintf(msg, 42, "hi");
                ^
src/unix/posix.h:53:34: note: expanded from macro 'p_snprintf'
                                 ^
/usr/include/secure/_stdio.h:57:73: note: expanded from macro 'snprintf'
  __builtin___snprintf_chk (str, len, 0, __darwin_obsz(str),
__VA_ARGS__)
```

Error messages should be sentence fragments, and therefore:

1. Should not begin with a capital letter,
2. Should not conclude with punctuation, and
3. Should not end a sentence and begin a new one