Added set_chain_policy() function, minor iptables binary name fix

Added set_chain_policy() function, minor iptables binary name fix

- Added set_chain_policy() function to allow built-in chain policies to be
  set to the specified target.  iptables/ip6tables does not allow the
  target to be set for non built-in chains.  Behind the scenes this
  function runs the command "iptables -t <table> -P <chain> <target>".
- Minor update to print the iptables binary name in 'croak' error
  conditions.  The binary name is either 'iptables' or 'ip6tables'.
- Minor perldoc updates to render links better (two spaces at the beginning
  of lines).