interface: support "zone" config option

interface: support "zone" config option

Many protocol handlers support "zone" option independently and they pass
it in the "data". Then it's read e.g. by a firewall[34].

Add support for "zone" directly to the netifd so:
1. It works for all protocols
2. Handlers don't have to duplicate code

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>