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.

include/signal.h

@@ -218,11 +218,8 @@ void (*sigset(int, void (*)(int)))(int);
 #define SIGSTKSZ 8192
 #endif
 
-#if defined(_XOPEN_SOURCE) || defined(_GNU_SOURCE)
-#define NSIG _NSIG
-#endif
-
 #if defined(_BSD_SOURCE) || defined(_GNU_SOURCE)
+#define NSIG _NSIG
 typedef void (*sig_t)(int);
 #endif