Implement operator 'vlan'

This pull-request depends on the parse-optional-int pull-request.

BPF code
```
(000) ldh      [12]
(001) jeq      #0x8100          jt 3    jf 2
(002) jeq      #0x9100          jt 3    jf 4
(003) ret      #65535
(004) ret      #0
```

Lua code:
```
return function(P,length)
   if not (14 <= length) then return false end
   local v1 = ffi.cast("uint16_t*", P+12)[0]
   if v1 == 129 then return true end
   do
      do return v1 == 145 end
   end
end
```