If too many channels are set up to be joined on startup, all of their
JOIN messages are sent at once, possibly triggering the flood protection
of the server and causing the link to be forcefully severed.  (In libera
I hit this limit with 12 channels - not a lot, all things considered.)

This patch mitigates this by sending a single JOIN message with all the
channels joined with ",".

This has a flaw, which is mentioned in the patch in a FIXME, in that if
the total length of the channels exceeds the maximum length of a message
then the message will likely split and break.  While checking for it
while joining the channel names is a possibility, it's probably better
if we split command messages more elegantly in the first place.