openat, save-cwd: avoid xmalloc

openat, save-cwd: avoid xmalloc

This removes a direct (but undocumented) dependency of openat on
xalloc, along with an indirect dependency via save-cwd.  It also
removes a dependency of save-cwd on xgetcwd, and thereby
indirectly on xalloc.  This change causes the openat substitute
to fall back on save_cwd when memory is tight, and for save_cwd to
fail instead of dying when memory is tight, but that's good enough.

* lib/openat-proc.c: Include stdlib.h (for malloc), not
xalloc.h (for xmalloc).
(openat_proc_name): Use malloc, not xmalloc.
* lib/save-cwd.c (save_cwd): Use getcwd, not xgetcwd.
* modules/save-cwd (Files): Depend on getcwd, not xgetcwd.