mwan3: Allow user to specify rules based on source interface

mwan3: Allow user to specify rules based on source interface

Add an option for adding rules based on source interface.
The default 0.0.0.0/0 src and destination ip addresses has been removed. It is unclear
how the 'any' family of rules would have worked, as it appears each rule always required an
ipv4 or ipv6 address src and destination address.  With this change, the any family will work
again.

I also cleaned up a bunch of repeated code around adding the iptables rules for
ipv4/ipv6/any in making the change.

Signed-off-by: Aaron Goodman <aaronjg@stanford.edu>