evaluate: improve error reporting in tproxy with inet family

evaluate: improve error reporting in tproxy with inet family

 # nft add rule inet filter divert ip daddr 0.0.0.0/0 meta l4proto tcp tproxy to :2000
 Error: conflicting protocols specified: ip vs. unknown. You must specify ip or ip6 family in tproxy statement
 add rule inet filter divert ip daddr 0.0.0.0/0 meta l4proto tcp tproxy to :2000
                             ~~~~~~~~                            ^^^^^^^^^^^^^^^

instead of:

 # nft add rule inet filter divert ip daddr 0.0.0.0/0 meta l4proto tcp tproxy to :2000
 Error: Conflicting network layer protocols.
 add rule inet filter divert ip daddr 0.0.0.0/0 meta l4proto tcp tproxy to :2000
                                                                 ^^^^^^^^^^^^^^^

Closes: https://bugzilla.netfilter.org/show_bug.cgi?id=1310
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>