Use lowercase for our error messages, per our custom.

Now that the function `git_stream__write_full` exists and callers of
`git_stream_write` have been adjusted, we can lift logic for short
writes out of the stream implementations. Instead, this is now handled
either by `git_stream__write_full` or by callers of `git_stream_write`
directly.

The callback functions that implement the `git_stream` structure are
only used inside of their respective implementation files, but they are
not marked as `static`. Fix this.

Our streams implementation takes a `size_t` that indicates the length of
the data buffer to be written, and returns an `ssize_t` that indicates
the length that _was_ written.  Clearly no such implementation can write
more than `SSIZE_MAX` bytes.  Ensure that each TLS stream implementation
does not try to write more than `SSIZE_MAX` bytes (or smaller; if the
given implementation takes a smaller size).

Follow up for PR #4917.

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

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

This change fixes a bunch of warnings that were discovered by compiling
with `clang -target=i386-pc-linux-gnu`. It turned out that the
intrinsics were not necessarily being used in all platforms! Especially
in GCC, since it does not support __has_builtin.

Some more warnings were gleaned from the Windows build, but I stopped
when I saw that some third-party dependencies (e.g. zlib) have warnings
of their own, so we might never be able to enable -Werror there.

Accept an enum (`git_stream_t`) during custom stream registration that
indicates whether the registration structure should be used for standard
(non-TLS) streams or TLS streams.

Update the new stream registration API to be `git_stream_register`
which takes a registration structure and a TLS boolean.  This allows
callers to register non-TLS streams as well as TLS streams.

Provide `git_stream_register_tls` that takes just the init callback for
backward compatibliity.