Wait for the xtables lock on all iptable calls

This fixes stepping-stone/libvirt-hooks#2

quoting the iptables man page:
Wait for the xtables lock.  To prevent multiple instances of the
program from running concurrently, an attempt will  be  made  to
obtain  an  exclusive  lock  at launch.  By default, the program
will exit if the lock cannot be obtained.  This option will make
the program wait until the exclusive lock can be obtained.