Set p_availableBuildings according to team, stage & class.
Buildings are listed in the order in which they're defined in enum buildable_t.

This is intended for use in bindings, e.g.
bind mwheelup "toggle building \$p_availableBuildings\$; build \$building\$"
bind mwheeldown "toggle - building \$p_availableBuildings\$; build \$building\$"
Team/area/private chat sound should be selected according to sender, not recipient.

This will mostly affect privileged spectators seeing non-spec teamchat.