perror: call strerror_r directly

perror: call strerror_r directly

No need to make a wrapper that burns static storage when we can
just use stack storage.

* modules/perror (Files): Drop strerror-impl.h.
* lib/perror.c (perror): Use our own stack buffer, rather than
calling a wrapper that uses static storage.
* doc/posix-functions/perror.texi (perror): Document a limitation
of our replacement.

Signed-off-by: Eric Blake <eblake@redhat.com>