Save netvars

Merge !1178 (merged) first.

This saves the value of netvars before joining netgames, then reverts when leaving. This should allow netvars to be saved by servers without client configs being affected.

As a client: When saving a config, the old value of a netvar is saved. When loading a config, changes to netvars apply after leaving the netgame.

These netvars are now saved:

dumpconsistency         teamscramble            respawnitem
allowjoin               scrambleonchange        flagtime
joinnextround           friendlyfire            tv_recycler
restrictskinchange      overtime                tv_teleporter
allowteamchange         pointlimit              tv_superring
startinglives           timelimit               tv_supersneaker
respawndelay            basenumlaps             tv_invincibility
competitionboxes        allowexitlevel          tv_jumpshield
allowseenames           inttime                 tv_watershield
matchboxes              coopstarposts           tv_ringshield
specialrings            cooplives               tv_forceshield
powerstones             advancemap              tv_bombshield
countdowntime           playersforexit          tv_1up
touchtag                exitmove                tv_eggmanbox
hidetime                pausepermission
autobalance             respawnitemtime

These previously only saved cvars are now netvars:

showjoinaddress         rejointimeout           maxsend                jointimeout     servername
maxplayers              resynchattempts         noticedownload         maxping
joindelay               blamecfail              downloadspeed          pingtimeout