Add setting for logging PMs on the server

One major hurdle with moderating an SRB2 server currently is that private messages aren't logged by the server. This allows bad actors to carry out abuse without being detected, since neither the host nor any of the moderators see the messages being sent over PM. To mitigate this problem, this patch adds a new variable hu_showprivatemessages that, when enabled, causes the server to start logging private messages over PM, allowing hosts to see private messages as they are being sent. This command has no effect on any other player, regardless if they're logged in as remote admins or not.

I also went ahead and cleaned up some of the chat logic, since cend was never used and it also had some unnecessary color bytes that had no effect.