Add _shadow variants to the two popular getpw functions (uid and nam).

This version of the function will always open the secure/shadow/master
password files. Soon, the regular variants of these functions will not.
(Intermixing shadow and regular gets a little weird; don't do that.)
Not using struct spwd and getspwnam functions to reduce churn in callers.
Should just be a one line diff in most places.
ok deraadt

src/include/pwd.h

@@ -1,4 +1,4 @@
-/*	$OpenBSD: pwd.h,v 1.23 2014/05/16 21:28:15 tedu Exp $	*/
+/*	$OpenBSD: pwd.h,v 1.24 2015/11/18 16:44:46 tedu Exp $	*/
 /*	$NetBSD: pwd.h,v 1.9 1996/05/15 21:36:45 jtc Exp $	*/
 
 /*-
@@ -93,6 +93,8 @@ struct passwd {
 __BEGIN_DECLS
 struct passwd	*getpwuid(uid_t);
 struct passwd	*getpwnam(const char *);
+struct passwd	*getpwuid_shadow(uid_t);
+struct passwd	*getpwnam_shadow(const char *);
 int		getpwnam_r(const char *, struct passwd *, char *, size_t,
 		    struct passwd **result);
 int		getpwuid_r(uid_t uid, struct passwd *, char *buf, size_t buflen,