Enable self message support for ZNC

Request the 'znc.in/self-message' capability to enable self message
support on ZNC.

Some servers (e.g. Bitlbee) assume self-message support; ZNC requires
a capability instead.  As self-message is already implemented,
there's little reason to not do this.

Who says we can't all get along? :)

There's more information in the IRCv3 commit that removed the
'self-message' capability.

See https://github.com/ircv3/ircv3-specifications/commit/1bfba47843c2526707c902034b3395af934713c8

Closes GH-202.