netinet: Do not forward or ICMP response to INADDR_ANY

The section 4 in the draft proposal [1] explicitly states that 0.0.0.0,
aka INADDR_ANY, retains its existing special meanings.

[1] https://datatracker.ietf.org/doc/draft-schoen-intarea-unicast-0

Reviewed by: glebius
Fixes: efe58855f3ea IPv4: experimental changes to allow net 0/8, 240/4, part of 127/8
MFC after: 5 days
Differential Revision: https://reviews.freebsd.org/D49157