changed cvar default setup code such that cvar default strings are locked after execu...

changed cvar default setup code such that cvar default strings are locked after executing default.cfg the first time (that is to say, their current values are copied into their default strings at this point), this fixes unsaved cvar problems in nexuiz caused by the default string being out of date on cvars that were modified by default.cfg
changed Reset To Defaults option in menu to use a newly added cvar_resettodefaults_all command before execing default.cfg to reset the binds

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@6454 d7cf8633-e32d-0410-b094-e92efae38249