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/