Core76: Drop output of iptables commands.

When updating a system with previously installed New Firewall,
the required firewall chains already are created. In this case error messages
will be displayed which could be ignored but may unsettle somebody.
Core 76: Fix sed command.

There was a syntax error in one of the sed commands, which produced the following error:
sed: -e expression #1, char 34: unknown option to `s'.
Make firewall convert scripts more robust.

The converter scripts procude a lot of error, when they get executed on a system with
a previously installed version of the New Firewall or they get run twice.

In this case the scripts will detect that their input files are missing and will exit with
an error message. The scripts now also check if the input files are empty (no corresponding
rules created) and will exit with an nothing to do message.
Drop "Extra Query" for Shutdown and Reboot from gui.cgi.

In the past this option has been used to prevent users from
accidently shutting down or rebooting their systems. With IPFire 2.15
there is a own subpage on the WUI for doing this actions, so there is no
need anymore for those extra query.