IRCv3: Update the signals so that handlers can inhibit the message from being displayed

This was mainly added for DCC neogotiation, but could be useful in other ways.

Testing Done:
Sent `CTCP VERSION` from irssi to a nick and a channel. Also sent `CTCP VERSION` from pidgin 3 to a nick and channel.

Bugs closed: PIDGIN-17721

Reviewed at https://reviews.imfreedom.org/r/2858/