stdlib: work around MirBSD WEXITSTATUS bug * lib/stdlib.in.h (includes): Guarantee WEXITSTATUS. * modules/stdlib (Depends-on): Add sys_wait. * tests/test-sys_wait.c (main): Enhance test. * tests/test-stdlib.c (main): Likewise. * doc/posix-headers/stdlib.texi (stdlib.h): Document the bug. Signed-off-by: Eric Blake <eblake@redhat.com>