Implement operator 'stp'

BPF code:
```
(000) ldh      [12]
(001) jgt      #0x5dc           jt 5    jf 2
(002) ldb      [14]
(003) jeq      #0x42            jt 4    jf 5
(004) ret      #65535
(005) ret      #0
```

Lua code:
```
return function(P,length)
   if not (15 <= length) then return false end
   local v1 = ffi.cast("uint16_t*", P+12)[0]
   local v2 = bit.rshift(bit.bswap(v1), 16)
   if not (v2 > 1500) then return false end
   do
      if not (v1 == 8) then return false end
      local v3 = P[14]
      do return v3 == 66 end
   end
end
```
I don't understand the check "v1 == 8", it doesn't seem to appear in the BPF code.