Add support for 802.1Q VLANs

[scaprile]

• VLAN 0 does not exist, so there's no need to do anything when VLANs are not used; the struct is initialized to 0 so when vlanid = 0, there is no 802.1Q tag added.
• Improves the L2 API (again), the header and trailer functions must be able to access the config in order to create the proper header/trailer fields and return the proper size; in this case the 802.1Q tag but there are other L2 that can have different formats
• Introduces the l2data member to ifc, in order to hold L2 specific configuration, and state (state is necessary in (for example, at least on) PPPoE, will be moved from statics to this struct in a future commit)
• Many changes are due to a rename: _footer --> _trailer and reformatting
• enable_crc_check --> enable_fcs_check. This one and enable_mac_check stay where they are, as they can be used by many L2 and this avoids duplication. However, some (weird) combinations of L2 over L2 might require having different configs; when that will be the case, they can be moved

Happily tested PPPoE on a VLAN