src: expose table flags

src: expose table flags

The nf_tables kernel API provides a way to disable a table using the
dormant flag. This patch adds the missing code to expose this feature
through nft.

Basically, if you want to disable a table and all its chains from seen
any traffic, you have to type:

 nft add table filter { flags dormant\; }

to re-enable the table, you have to:

 nft add table filter

this clears the flags.

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