xtables-restore: Introduce struct nft_xt_restore_state

xtables-restore: Introduce struct nft_xt_restore_state

This data structure holds parser state information. A follow-up patch
will extract line parsing code into a separate function which will need
a place to persistently store this info in between calls.

While being at it, make 'in_table' variable boolean and drop some extra
braces in conditionals checking its value.

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