smtpd: allow arguments on NOOP

smtpd: allow arguments on NOOP

per RFC3521 § 4.1.1.9 the NOOP command allows optionally one argument
that SHOULD be ignored.  For semplicity, relax it to allow anything after it.

Original diff by Sebastian J. Bronner, GitHub PR 1150, tweaked by me to
add smtp_check_noop().

Gilles agrees, ok millert@