fix MPLS code generation:

fix MPLS code generation:

if we have a MPLS label stack deeper > 1 then generate a match
  for a cleared bottom-of-stack-bit of the previous MPLS shim header
  rather than just incrementing the offset;

if there is a compined expression of MPLS and IP like e.g.
  "mpls && ip" | "mpls && ip host" | "mpls && ip src net"
  then poison the linkoffset to make sure that other code generators
  do not try to match link-layer protos like Q_ARP, Q_RARP etc.

introduce a new function gen_null() that matches against the first nibble
  of the IP header and matches if the bottom-of-stack bit is set;

TODO: IPv6 stuff i.e. gen_host6() etc.