monitors/alarm/command:on now takes an optional cause (query or POST data)
and passes it to zmu --cause, so the event records it in place of
"Forced Web". Without a cause nothing changes.
The cause is stored as the event's Cause and in its Notes, so alarmCause()
keeps it to plain text: letters, digits, spaces and plain punctuation, cut
to the 31 bytes zmu stores on a character boundary, falling back to the
default when nothing is left. The value is quoted with escapeshellarg()
for the zmu command line.
Test: tests/php/test_monitor_alarm_cause.php
fixes#5348