ebtables-nft: add broute table emulation

ebtables-nft: add broute table emulation

Use new 'meta broute set 1' to emulate -t broute.  If '-t broute' is given,
automatically translate -j DROP to 'meta broute set 1 accept' internally.

Reverse translation zaps the broute and pretends verdict was DROP.

Note that BROUTING is internally handled via PREROUTING, i.e. 'redirect'
and 'nat' targets are not available, they will need to be emulated via
nft expressions.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Phil Sutter <phil@nwl.cc>