Add command to allow adding/changing/removing rcon password

[GooberRF]

This PR adds the server_rcon_password command, which can be used to add an rcon password to a server without one, change the existing rcon password to something else, or remove the rcon password entirely.

Syntax is server_rcon_password <password>

[is-this-c]

Like sv_pass I think sv_rcon_pass is way better. It is just simpler.