Implement core IRC capability handling and "CAP" command

Implement core IRC capability handling and "CAP" command

This patch implements the core functions to support "IRC Capabilities"
and the IRC "CAP" command as used by other servers and specified here:
<http://www.leeh.co.uk/draft-mitchell-irc-capabilities-02.html>.

It enables ngIRCd to support the defined handshake, but it doesn't
implement any capabilities, so "CAP LS" and "CAP LIST" always return
the empty set and "CAP REQ ..." always fails with "CAP NAK".