Fix stat, add fstat and other NBSD libc fixes.

This patch changes the NBSD libc stat implemenation and adds
fstat (and headers), taken from current libc.
It also adds weaks alias to functions in the resolver that
were removed from public use in NetBSD but that are still
used by Minix, and fixes a NetBSD non-REENTRANT bug in
in gen/initdir.c.

lib/nbsd_libc/gen/initdir.c

@@ -39,6 +39,7 @@ __RCSID("$NetBSD: initdir.c,v 1.1 2010/09/26 02:26:59 yamt Exp $");
 #ifdef __minix
 /* NetBSD BUG on !_REENTRANT */
 #include <sys/cdefs.h>
+#include <sys/featuretest.h>
 #include <sys/types.h>
 #endif