irc/quassel: split into two ports (client and server)

irc/quassel-core provides the server back-end.
irc/quassel provides the client front-end.
irc/quassel can also be built with the MONO option enabled to also
install the client and server combined into a single monolithic binary.

Submitted by: Chris Rees <crees@physics.org> (in part)
Approved by: adamw (mentor, implicit)