Add vlan 802.1q/802.1ad support as netifd devices

At moment netifd supports just 802.1q vlan, you can configure them using a concise but "hacky"
syntax using an interface config section, with this patch netifd acquire the capability
of configuring 802.1ad and 802.1q vlan using config device sections, so you can define a vlan device
plus interface with something like this:

config device 'test'
        option type '8021ad'
        option name 'test'
        option ifname 'eth0'
        option vid '1000'

config interface 'testif'
        option ifname 'test'
        option proto 'none'
        option auto '1'

old syntax for 802.1q keeps working so no retrocompatibility problems,
to keep retrocompatibility means also that user must not use name/ifname like eth0.2
for devices declared with the new style because this would trigger the "old style"
when interface config section is parsed

Signed-off-by: Gioacchino Mazzurco <gmazzurco89@gmail.com>