netfilter: nftables: introduce table ownership

A userspace daemon like firewalld might need to monitor for netlink
updates to detect its ruleset removal by the (global) flush ruleset
command to ensure ruleset persistency. This adds extra complexity from
userspace and, for some little time, the firewall policy is not in
place.

This patch adds the NFT_TABLE_F_OWNER flag which allows a userspace
program to own the table that creates in exclusivity.

Tables that are owned...

- can only be updated and removed by the owner, non-owners hit EPERM if
  they try to update it or remove it.
- are destroyed when the owner closes the netlink socket or the process
  is gone (implicit netlink socket closure).
- are skipped by the global flush ruleset command.
- are listed in the global ruleset.

The userspace process that sets on the NFT_TABLE_F_OWNER flag need to
leave open the netlink socket.

A new NFTA_TABLE_OWNER netlink attribute specifies the netlink port ID
to identify the owner from userspace.

This patch also updates error reporting when an unknown table flag is
specified to change it from EINVAL to EOPNOTSUPP given that EINVAL is
usually reserved to report for malformed netlink messages to userspace.

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