chmode: end the grace period more intelligently (#84)

chmode: end the grace period more intelligently (#84)

We were ending the flood grace period for any channel mode command other
than `MODE #foo [bq]` by means of a hardcoded check. I've moved that to
after we parse the mode string, so we can correctly identify all
requests to change modes and end the grace period on exactly those.

It would have been entirely possible to move the check even further down
and flood_endgrace on only mode commands that *actually* change modes,
but I don't like the idea of making it sensitive to external conditions.