fix multiple minor namespace issues in headers

fix multiple minor namespace issues in headers

fcntl.h: AT_* is not a reserved namespace so extensions cannot be
exposed by default.

langinfo.h: YESSTR and NOSTR were removed from the standard.

limits.h: NL_NMAX was removed from the standard.

signal.h: the conditional for NSIG was wrongly checking _XOPEN_SOURCE
rather than _BSD_SOURCE. this was purely a mistake; it doesn't even
match the commit message from the commit that added it.