perror: Avoid clobbering the strerror buffer when possible.

perror: Avoid clobbering the strerror buffer when possible.

* lib/strerror-impl.h: New file, extracted from lib/strerror.c.
* lib/strerror.c: Include it.
* modules/strerror (Files): Add lib/strerror-impl.h.
* lib/perror.c: Include <stdlib.h>, intprops.h, verify.h.
(my_strerror): New function, defined through lib/strerror-impl.h.
(perror): Use it instead of strerror.
* modules/perror (Files): Add lib/strerror-impl.h.
(Depends-on): Remove strerror. Add intprops, verify, strerror_r-posix.