REQ: Better support for mvdsv's reliable sounds feature

[meag]

Currently players need to add /setinfo rsnd 0 to disable reliable sounds when connecting to mvdsv server.

At high ping, the reliable sounds will be confirmed later and so rapid-firing (sng for example) will be doubled up in the reliable packet, over-writing each other client-side and leading to the sng sounding like it is firing far slower.

Either:

• add a cvar that is mirrored in userinfo, so players can choose to have this enabled/disabled as required.
• do something clever to detect the doubled-up firing from previous reliable packet and treat accordingly.