chown: fix user:group option parsing

Check that either user or group is correctly passed, and adapt
documention according to that.

chown.c

@@ -40,8 +40,9 @@ chownpwgr(const char *path, struct stat *st, void *data, struct recursor *r)
 static void
 usage(void)
 {
-	eprintf("usage: %s [-h] [-R [-H | -L | -P]] [owner][:[group]] "
-	        "file ...\n", argv0);
+	eprintf("usage: %s [-h] [-R [-H | -L | -P]] owner[:[group]] file ...\n"
+	        "       %s [-h] [-R [-H | -L | -P]] :group file ...\n",
+	        argv0, argv0);
 }
 
 int
@@ -99,6 +100,9 @@ main(int argc, char *argv[])
 			gid = estrtonum(group, 0, UINT_MAX);
 		}
 	}
+	if (uid == (uid_t)-1 && gid == (gid_t)-1)
+		usage();
+
 	for (argc--, argv++; *argv; argc--, argv++)
 		recurse(*argv, NULL, &r);