nftables-ctl: load rules in determinitic order

- handle embedded white space in file names by using null characters as file separator
- sort rules to allow a deterministic loading
- only process files
- also load rules not ending with .rules (as used by current userland ntftables tools)
nftables-ctl: only use rules with the rules extension
Merge pull request #2 from Mic92/patch-2

nftables-ctl: load rules in determinitic order