vpnmain.cgi: Fixes bug13737 - revoke any deleted client certificate

vpnmain.cgi: Fixes bug13737 - revoke any deleted client certificate

- As the serial number is incremented now for each new cert that is created, then when a
   client cert is deleted from the ipsec list in the wui then that cert must be revoked
   otherwise it will still be listed in the .index file as a valid certificate and then
   the certificate name and DN could never be used again.
- Running the revoke command when deleting a client cert leaves the details in the .index
   file but the same name can then be re-used and will get a new serial number etc.

Fixes: bug13737
Tested-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>