iptables.c: lock the xtables.lock

iptables.c: lock the xtables.lock

When using fw3 together with other applications or scripts a race
conditions might occur. When fw3 is preparing the new tables, another
application can use the executable `iptables` which modifies the
kernel-tables.  libxtables will notify this and fails when fw3 is
committing the changes resulting in a failed firewall.

Now waits in a while loop until the lock is gone, activate the lock
itself and applies changes.

To reproduce the bug the following two scripts should run in parrallel,
after a few seconds the latter stop and leaves a broken firewall:

    while true; do iptables -N locking; done

and

    while [ "$(iptables -w -L OUTPUT | wc -l)" -gt 2 ]; do fw3 reload; done

The following message will appear

        Warning: iptc_commit(): Resource temporarily unavailable

and connectivity is gone.

Tested in an LXC and Qemu container.

Signed-off-by: Alexander Couzens <lynxis@fe80.eu>
[fixed waiting for unlock and commit message]
Signed-off-by: Paul Spooren <mail@aparcar.org>