libgit2 has two distinct requirements that were previously solved by
`git_buf`.  We require:

1. A general purpose string class that provides a number of utility APIs
   for manipulating data (eg, concatenating, truncating, etc).
2. A structure that we can use to return strings to callers that they
   can take ownership of.

By using a single class (`git_buf`) for both of these purposes, we have
confused the API to the point that refactorings are difficult and
reasoning about correctness is also difficult.

Move the utility class `git_buf` to be called `git_str`: this represents
its general purpose, as an internal string buffer class.  The name also
is an homage to Junio Hamano ("gitstr").

The public API remains `git_buf`, and has a much smaller footprint.  It
is generally only used as an "out" param with strict requirements that
follow the documentation.  (Exceptions exist for some legacy APIs to
avoid breaking callers unnecessarily.)

Utility functions exist to convert a user-specified `git_buf` to a
`git_str` so that we can call internal functions, then converting it
back again.

Make p_getcwd match the rest of our win32 path handling semantics.

(This is currently only used in tests, which is why this disparity went
unnoticed.)

Make some syntax change to follow coding style.

GCC C11 warnings

Instead of buf->"typeofbuffer"ReparseBuffer the members will be
referenced with buf->ReparseBuffer."typeofbuffer"

https://docs.microsoft.com/en-us/windows-hardware/drivers/ddi/ntifs/ns-ntifs-_reparse_data_buffer?redirectedfrom=MSDN
calls the union DUMMYUNIONNAME but that looks a bit cluttered.

Introduce GIT_WARN_UNUSED_RESULT

Previously, the location of `GIT_WARN_UNUSED_RESULT` was causing it to
be included _after_ a bunch of other headers (namely `src/vector.h`),
which broke the build.

This change does two things:

* Moves the `GIT_WARN_UNUSED_RESULT` above most of the `$include`s in
  `src/common.h`.
* Stops including `vector.h` from `src/win32/path_w32.c` since the
  header itself does not use it.

Even on systems without C99 where long long and stdint are both
missing, we can shim stdint and point it to any compiler-specific
type (i.e long long, _int64, etc.).

Also next is constant suffixes and determining what needs to
include stdint.