Add support for locking execution, and prevent multiple instances from competing

This should fix an issue where concurrent execution of iptables ends up in a deadlock.
Add a timer signal to the lock operation, such that it doesn't block forever
Merge pull request #8 from jvehent/master

Add support for locking when executing commands as a workaround for concurrent access to netfilter resources. In order to avoid this situation in all cases, nftables (newer kernel/netfilter) is required.