channel modes: only handle MAX_CMODES_ARG modes with arguments

Limit the MODE command to handle a maximum of MAX_CMODES_ARG (5) channel
modes that require an argument (+Ibkl) per call.

Please note: Further modes that require arguments are silently ignored
and end the handling of any further modes.
This is similar to the behavior of ircd2.11 (silently ignores but seems
to handle other modes) as well as ircd-seven (silently ignores but handles
some(!) other modes) ...