parser_bison: explicit indication on export ruleset

parser_bison: explicit indication on export ruleset

This patch modifies the grammar to explicitly indicate what you want to
export, eg.

 # nft export ruleset json

This leaves room to extend this later on to support other object types,
such as integrating conntrack into nft.

This also leaves the syntax in consistent state wrt. other existing
objects. The existing syntax is still preserved.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>