ebtables: Refuse unselected targets' options

ebtables: Refuse unselected targets' options

Unlike legacy, ebtables-nft would allow e.g.:

| -t nat -A PREROUTING --to-dst fe:ed:00:00:ba:be

While the result is correct, it may mislead users into believing
multiple targets are possible per rule. Better follow legacy's behaviour
and reject target options unless they have been "enabled" by a previous
'-j' option.

To achieve this, one needs to distinguish targets from watchers also
attached to 'xtables_targets' and otherwise behaving like regular
matches. Introduce XTABLES_EXT_WATCHER to mark the two.

The above works already, but error messages are misleading when using
the now unsupported syntax since target options have been merged
already. Solve this by not pre-loading the targets at all, code will
just fall back to loading ad '-j' parsing time as iptables does.

Note how this also fixes for 'counter' statement being in wrong position
of ebtables-translate output.

Fixes: fe97f60e5d2a9 ("ebtables-compat: add watchers support")
Signed-off-by: Phil Sutter <phil@nwl.cc>