Bugfixes, API additions, start of mods

Fixed a nasty, awful SIGSEGV bug caused by the way that the signal
lock/condvar are loaded/unloaded into/out of the Connection class.

Fleshed out the data and functionality the Server class exposes.  Still
need to add MotD.

Started work on mod support.  Can be seen between src/ping/main.cpp,
src/server_mod_loader.cpp, and include/mod.hpp.  I'm probably going to
end up writing a purpose-built collection for storing the mods, but for
now a vector of tuples does the job acceptably.

Encountered an odd problem where Windows wouldn't properly load the
address of a const char *, so I made the faculty that retrieves the
mod's name into a function rather than a variable.  I will try and
revisit this however, since it's bugging me.

Updated the send thread so that it uses polling, which means that it can
handle all connections that need attention in 1-2 passes rather than
2-4.