Extensions to the netmap framework - Week 10

[Stefano Duo]

Hi,

during this past week i modified VALE vlan so that it is aware of buffer
offsets.
The lookup function now checks if the new `struct nm_bdg_fwd` field
`ft_head_space` is longer that the space needed for the vlan tag; if it is
that space is used to store the tag, otherwise the packet is moved forward.

I also improved the per-ring fixed buffer documentation.

Project wiki:
https://wiki.freebsd.org/SummerOfCode2018Projects/ExtensionsToNetmap
SVN repository: https://svnweb.freebsd.org/socsvn/soc2018/sduo/

Stefano.