Our file utils functions all have a "futils" prefix, e.g.
`git_futils_touch`. One would thus naturally guess that their
definitions and implementation would live in files "futils.h" and
"futils.c", respectively, but in fact they live in "fileops.h".

Rename the files to match expectations.

`cvar` is an unhelpful name.  Refactor its usage to `configmap` for more
clarity.

Since we were not expecting this config entry to contain a string, we
would fail as soon as its (cached) value would be accessed. Hence,
provide some constants for the 4 states we use, and account for "always"
when we decide to reflog changes.

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.

Callers of `git_config__cvar` already handle the case where the
function returns an error due to a failed configuration variable
lookup, but we are actually swallowing errors when calling
`git_config__lookup_entry` inside of the function.

Fix this by returning early when `git_config__lookup_entry`
returns an error. As we call `git_config__lookup_entry` with
`no_errors == false` which leads us to call `get_entry` with
`GET_NO_MISSING` we will not return early when the lookup fails
due to a missing entry. Like this we are still able to set the
default value of the cvar and exit successfully.

This changes the get_entry() method to return a refcounted version of
the config entry, which you have to free when you're done.

This allows us to avoid freeing the memory in which the entry is stored
on a refresh, which may happen at any time for a live config.

For this reason, get_string() has been forbidden on live configs and a
new function get_string_buf() has been added, which stores the string in
a git_buf which the user then owns.

The functions which parse the string value takea advantage of the
borrowing to parse safely and then release the entry.

Validate HFS ignored char ".git" paths when `core.protectHFS` is
specified.  Validate NTFS invalid ".git" paths when `core.protectNTFS`
is specified.

And decrease extra reload checks of config data.