Implement built-in server for singleplayer/listen/dedicated servers

[eira-fransham]

Motivation

Currently, we can only use external servers. A lot of the most-compelling usecases for Seismon are blocked on it being able to run its own game logic. For example, being able to write Quake-compatible mods in languages other than QuakeC. The work was started by Cormac O'Brien in Richter, but it is very unfinished and it is one of the parts of Richter that have more-or-less not been touched at all.

Proposal

Continue working on the server, implementing unimplemented functions. A lot of the groundwork has been done already, we just need to keep working on it.