xtables: implement ebtables-{save,restore}

xtables: implement ebtables-{save,restore}

The code for ebtables-restore was derived from legacy code,
ebtables-save is actually a new implementation using the existing
infrastructure and trying to adhere to legacy perl script output
formatting as much as possible.

This introduces a new format flag (FMT_EBT_SAVE) to allow
nft_bridge_save_rule() to distinguish between ruleset listing (i.e.,
ebtables -L) and saving via ebtables-save - the two differ in how
counters are being formatted. Odd, but that's how it is.

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