free: preserve errno [BZ#17924]

In the next release of POSIX, free must preserve errno
<https://www.austingroupbugs.net/view.php?id=385

>.
Modify __libc_free to save and restore errno, so that
any internal munmap etc. syscalls do not disturb the caller's errno.
Add a test malloc/tst-free-errno.c (almost all by Bruno Haible),
and document that free preserves errno.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>