minor README typo fixes
[libfko] apply zero_buf() to stack allocated Rijndael context for encrypt/decrypt
[server] Account for older versions of iptables that don't have -C

This commit updates fwknopd to test for the existance of the iptables '-C'
rule checking functionality since older versions of iptables don't have this.
If it isn't offered by the installed version of iptables, then revert to parsing
fwknop chains to see if iptables rules already exist before adding new rules (to
avoid duplicates).