vpnmain.cgi: Fix editing connections that are using a PSK

vpnmain.cgi: Fix editing connections that are using a PSK

This patch takes care of properly decoding the PSK if it was already
stored base64-encoded. If the connection is edited, it always will be
stored base64-encoded upon save.

It would have been nice to not send the PSK back to the browser again
(although the security benefits would have been marginal), but that
would make the code even messier than it is.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>