src: Reject 'export vm json' command

src: Reject 'export vm json' command

Since libnftnl recently dropped JSON output support, this form of JSON
export is not available anymore. Point at 'nft -j list ruleset' command
for a replacement in error message.

Since 'export' command is not useable anymore, remove it from
documentation. Instead point out that 'list ruleset' command serves well
for dumping and later restoring.

To not cause pointless inconvenience for users wishing to store their
ruleset in JSON format, make JSON parser fallback to CMD_ADD if no
recognized command property was found. This allows to feed the output of
'nft -j list ruleset' into 'nft -f' without any modification.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>