Update users' "present" and "seen" fields when unsuspending a channel.

Update users' "present" and "seen" fields when unsuspending a channel.

Bug reported by ZeRoFiGhter.

src/chanserv.c (chanserv_expire_suspension): Add some comments.  Add a loop
    to update presence information for anyone in the channel at the time
    the channel is unsuspended.