Add uid_from_user() and gid_from_group(), derived from pax's cache.c.

It replaces the existing pwcache.c functions user_from_uid(3) and
group_from_gid(3) with the pax equivalents.  Adapted from NetBSD
(mycroft) changes from our own pax's cache.c.  OK guenther@

src/include/grp.h

@@ -1,4 +1,4 @@
-/*	$OpenBSD: grp.h,v 1.12 2014/08/31 04:04:38 guenther Exp $	*/
+/*	$OpenBSD: grp.h,v 1.13 2018/09/13 12:31:15 millert Exp $	*/
 /*	$NetBSD: grp.h,v 1.7 1995/04/29 05:30:40 cgd Exp $	*/
 
 /*-
@@ -70,7 +70,8 @@ int		 getgrnam_r(const char *, struct group *, char *,
 #endif
 #if __BSD_VISIBLE
 int		 setgroupent(int);
-char		*group_from_gid(gid_t, int);
+int		 gid_from_group(const char *, gid_t *);
+const char	*group_from_gid(gid_t, int);
 #endif
 __END_DECLS
 

src/include/pwd.h

@@ -1,4 +1,4 @@
-/*	$OpenBSD: pwd.h,v 1.25 2017/03/09 10:13:03 fcambus Exp $	*/
+/*	$OpenBSD: pwd.h,v 1.26 2018/09/13 12:31:15 millert Exp $	*/
 /*	$NetBSD: pwd.h,v 1.9 1996/05/15 21:36:45 jtc Exp $	*/
 
 /*-
@@ -106,7 +106,8 @@ void		 endpwent(void);
 #endif
 #if __BSD_VISIBLE
 int		 setpassent(int);
-char		*user_from_uid(uid_t, int);
+int		 uid_from_user(const char *, uid_t *);
+const char	*user_from_uid(uid_t, int);
 char		*bcrypt_gensalt(u_int8_t);
 char		*bcrypt(const char *, const char *);
 int		bcrypt_newhash(const char *, int, char *, size_t);