ebtables: Support '-p Length'

ebtables: Support '-p Length'

To match on Ethernet frames using the etherproto field as length value,
ebtables accepts the special protocol name "LENGTH". Implement this in
ebtables-nft using a native match for 'ether type < 0x0600'.

Since extension 802_3 matches are valid only with such Ethernet frames,
add a local add_match() wrapper which complains if the extension is used
without '-p Length' parameter. Legacy ebtables does this within the
extension's final_check callback, but it's not possible here due for lack of
fw->bitmask field access.

While being at it, add xlate support, adjust tests and make ebtables-nft
print the case-insensitive argument with capital 'L' like legacy
ebtables does.

Signed-off-by: Phil Sutter <phil@nwl.cc>