nft-ruleparse: Introduce nft_create_target()

nft-ruleparse: Introduce nft_create_target()

Like nft_create_match(), this is a small wrapper around the typical
target extension lookup and (standard) init code.

To use it from nft_parse_target() and nft_parse_log(), introduce an
inner variant which accepts the target payload size as parameter.

The call to rule_parse_ops::target callback was problematic with
standard target, because the callbacks initialized
iptables_command_state::jumpto with the target name, "standard" in that
case. Perform its tasks in nft_create_target(), keep it only for bridge
family's special handling of watcher "targets".

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