Add --compat option to *tables-nft and *-nft-restore commands

Add --compat option to *tables-nft and *-nft-restore commands

The flag sets nft_handle::compat boolean, indicating a compatible rule
implementation is wanted. Users expecting their created rules to be
fetched from kernel by an older version of *tables-nft may use this to
avoid potential compatibility issues.

Changes since v1:
- Expect short option '-C' in {ip,ip6,eb}tables-nft-restore command line
  parser
- Support -C/--compat in arptables-nft-restore, too
- Update man pages with the new flag

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