Fix multiplayer in game joining

[JohnAFernandez]

Again, creating early in draft since my dev environment is not really available.

In game joining has been on a long journey. Volition added most of the original code back in 1999. Since then, it has only been worked on sparsely for the past few years. Other multiplayer priorities, like making sure you can hit anything, have always superceded it.

With this, players will be able to join any multiplayer game that is in the game play state.

[notimaginative]

Had a (very) quick look at the boost version of the bitset stuff and I think it might be better to implement a to_block()/from_block() for our bit set instead of using the string method. Basically just goes to/from a vector of single bytes: for count of bitset.size() divided by 7, apply set flags to a byte and push to vector. For multi we'd write a single byte for the vector size, then each byte in the vector, giving 7*255 total supported flags per bitset. Long term that should work much better given the space constraints of multi packets.

[JohnAFernandez]

Closing since work continued on another branch.