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
Reviewed by: trasz
Sponsored by: Axcient
Differential Revision: https://reviews.freebsd.org/D30734

(cherry picked from commit 4a06e9377398b34922f8a67d7cb3ea980b95bde8)