nft: Make table creation purely implicit

nft: Make table creation purely implicit

While asserting a required builtin chain exists, its table is created
implicitly if missing. Exploit this from xtables-restore, too: The only
actions which need adjustment are chain_new and chain_restore, i.e. when
restoring (either builtin or custom) chains.

Note: The call to nft_table_builtin_add() wasn't sufficient as it
doesn't set the table as initialized and therefore a following call to
nft_xt_builtin_init() would override non-default base chain policies.

Note2: The 'table_new' callback in 'nft_xt_restore_cb' is left in place
as xtables-translate uses it to print an explicit 'add table' command.

Note3: nft_table_new() function was already unused since a7f1e208cdf9c
("nft: split parsing from netlink commands").

Signed-off-by: Phil Sutter <phil@nwl.cc>