Fix key lookup in ipfw(8) broken since r232865.
Print warning for IPv4 address strings which are valid in
inet_aton() but not valid in inet_pton(). (1)

Found by: Özkan KIRIK <ozkan.kirik@gmail.com>
Submitted by: Ian Smith <smithi@nimnet.asn.au> (1)
MFC after: 2 weeks
Sponsored by: Yandex LLC