A bunch of bugfixes about too many wads in netgames

Fixes:

• Having bunch of music WADs added and then breaching the file limit(s) through joining a server with important files added. http://mb.srb2.org/showthread.php?t=42662
• Having a bunch of music WADs added, joining a server, and then breaching the file limit(s) when the server adds a file. https://mb.srb2.org/showthread.php?t=34664
• Informing the adminplayer whenever they try to send a request addfile net command but the file limit(s) are reached.

Done into next because I might as well be 100% safe, even though this could probably work on master as well.