Don't send multiple error messages in response to a single command

Don't send multiple error messages in response to a single command

While a few commands (such as HELP and NOOP) are allowed in between
USER and PASS, most of the others are not.  However if such command
is issued, ftpd should abort the command evaluation early and reply
with "530 Please login with USER and PASS."

From form@, with input from deraadt@ and OK millert@