We require the library to be initialized with git_libgit2_init before it
is functional.  However, if a user tries to uses the library without
doing so - as they might when getting started with the library for the
first time - we will likely crash.

This commit introduces some guard rails - now instead of having _no_
allocator by default, we'll have an allocator that always fails, and
never tries to set an error message (since the thread-local state is
set up by git_libgit2_init).  We've modified the error retrieval
function to (try to) ensure that the library has been initialized before
getting the thread-local error message.

(Unfortunately, we cannot determine if the thread local storage has
actually been configured, this does require initialization by
git_libgit2_init.  But a naive attempt should be good enough for most
cases.)

Our "global initialization" has accumulated some debris over the years.
It was previously responsible for both running the various global
initializers (that set up various subsystems) _and_ setting up the
"global state", which is actually the thread-local state for things
like error reporting.

Separate the thread local state out into "threadstate".  Use the normal
subsystem initialization functions that we already have to set it up.
This makes both the global initialization system and the threadstate
system simpler to reason about.

When compiling libgit2 with -DDEPRECATE_HARD, we add a preprocessor
definition `GIT_DEPRECATE_HARD` which causes the "git2/deprecated.h"
header to be empty. As a result, no function declarations are made
available to callers, but the implementations are still available to
link against. This has the problem that function declarations also
aren't visible to the implementations, meaning that the symbol's
visibility will not be set up correctly. As a result, the resulting
library may not expose those deprecated symbols at all on some platforms
and thus cause linking errors.

Fix the issue by conditionally compiling deprecated functions, only.
While it becomes impossible to link against such a library in case one
uses deprecated functions, distributors of libgit2 aren't expected to
pass -DDEPRECATE_HARD anyway. Instead, users of libgit2 should manually
define GIT_DEPRECATE_HARD to hide deprecated functions. Using "real"
hard deprecation still makes sense in the context of CI to test we don't
use deprecated symbols ourselves and in case a dependant uses libgit2 in
a vendored way and knows it won't ever use any of the deprecated symbols
anyway.

Stop returning a void for functions, future-proofing them to allow them
to fail.

The old POSIX regex wrappers have been superseded by our own regexp API
that provides a higher-level abstraction. Remove the POSIX wrappers in
favor of the new one.

Right now, we only provide a `git_error_set` that has a variadic
function signature. It's impossible to drive this function in a
C89-compliant way from other functions that have a variadic
signature, though, like for example `git_parse_error`.

Implement a new `git_error_vset` function that gets a `va_list`
as parameter, fixing the above problem.

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.

Signed-off-by: Sven Strickroth <email@cs-ware.de>

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