userpreferences.php: minor changes
-changed WIDTHERR to RESERR
-changed HEIGHTERR to AUDIOERR
-modified userpreferences: added 6 resolutions to resolutionArray; added printSubmitErr(RESERR); added printSubmitErr(AUDIOERR)
-modified processUserPrefsInput: added input validation to resolution and audiomode
-modified togglePubKeys: added RESERR and AUDIOERR to conditions to show the RDP Preferences pane