No default secondary weapon option in launcher

[BranDougherty]

As pointed out in opensoldat/opensoldat#165, we should have some UI for cl_player_secwep, which is an integer cvar in the range [0..3], allowing the user to pick either the USSOCOM, Combat Knife, Chainsaw, or M72 LAW as their default secondary weapon when entering a server.

Could be a dropdown menu, in Settings > Game > Default secondary weapon, like right underneath Allow server mods. It doesn't even really need icons for a first draft. Or we could add something in Settings > Player, not sure which is better.