virtual interface: vlan

[Savagedlight]

Add support for manipulating virtual interface 'vlan'.

• assign parent interface
• assign VLAN ID
• Other? Check ifconfig's ifvlan.c

Consider adding a method for creating VLAN interface and assigning parent interface + VLAN id in one go.

[Savagedlight]

Creating vlan interfaces are now supported through two methods:

• ifconfig_create_interface_vlan(...) which also configures the newly created interface with parent device and vlan tag.
• ifconfig_set_vlantag(...) which takes an existing, unconfigured vlanN interface and configures it with a parent interface and vlan tag.