stdio, unistd: guarantee ssize_t

stdio, unistd: guarantee ssize_t

Per POSIX 2008, ssize_t needs to exist whether or not we are
replacing functions that use it.  For now, we only guarantee
it if we know that the user expects a function that uses
that type, while waiting for glibc to catch up to POSIX.

* lib/unistd.in.h (includes): Ensure that types required by POSIX
2008 are exposed when needed.
* lib/stdio.in.h (includes): Likewise.
Reported by Ralf Wildenhues.

Signed-off-by: Eric Blake <ebb9@byu.net>
(cherry picked from commit 60ad192b7032409b579ea059591cfd2558f57075)