id: sanitize arguments better

The -[AMc] flags ignore the user argument.  Better if id rejects
invocations that include a user argument along with any of those flags.

PR:		256554
MFC after:	2 weeks
Reviewed by:	trasz
Sponsored by:	Axcient
Differential Revision: https://reviews.freebsd.org/D30734

usr.bin/id/id.c

@@ -144,6 +144,8 @@ main(int argc, char *argv[])
 
 	if (iswhoami && argc > 0)
 		usage();
+	if ((cflag || Aflag || Mflag) && argc > 0)
+		usage();
 
 	switch(Aflag + Gflag + Mflag + Pflag + gflag + pflag + uflag) {
 	case 1: