The information about the type of a revision spec is not information
about the parser.  Name it accordingly, so that `git_revparse_mode_t`
is now `git_revspec_t`.  Deprecate the old name.

Random code cleanups and fixes

We've accumulated quite some functions which are never used outside of
their respective code unit, but which are lacking the `static` keyword.
Add it to reduce their linkage scope and allow the compiler to optimize
better.

When extracting curly braces (e.g. the "upstream" part in
"HEAD@{upstream}"), we put the curly braces' contents into a `git_buf`
structure, but don't check the return value of `git_buf_putc`. So when
we run out-of-memory, we'll use a partially filled buffer without
noticing.

Let's fix this issue by checking `git_buf_putc`'s return value.

The old POSIX regex API has been superseded by our new regexp API.
Convert all users to make use of the new one.

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.