* lib/c-stack.c: Include unistd.h unconditionally, since we now assume

the unistd module.
* lib/getlogin_r.c: Likewise.
* lib/getlogin_r.h: Likewise.
* lib/glob.c: Likewise.
* lib/pagealign_alloc.c: Likewise.
* lib/unistd_.h: Remove; no longer needed.
* modules/c-stack (Depends-on): Add unistd.
* modules/getlogin_r: Likewise.
* modules/glob: Likewise.
* modules/pagealign_alloc: Likewise.
* modules/unistd (Files): Remove lib/unistd_.h.
(EXTRA_DIST): Remove.
(unistd.h): Create using 'echo' rather than 'cp', so that we don't
need unistd_.h.
(MOSTLYCLEANFILES): Remove unistd.h-t.