src: Add import command for low level json

src: Add import command for low level json

This new operation allows to import low level virtual machine ruleset in
json to make incremental changes using the parse functions of libnftnl.

A basic way to test this new functionality is:

 $ cat file.json | nft import vm json

where the file.json is a ruleset exported in low level json format.

To export json rules in low level virtual machine format
we need to specify "vm" token before json. See below
        $ nft export vm json

and
        $ nft export/import json

will do no operations.
Same goes with  "$nft monitor"

Highly based on work from  Alvaro Neira <alvaroneay@gmail.com>
and Arturo Borrero <arturo@netfilter.org>

Acked-by: Arturo Borrero Gonzalez <arturo@netfilter.org>
Signed-off-by: Shyam Saini <mayhs11saini@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>