always widen the imputed netmask if it is narrower than the specified octets.
fixes a strange behaviour where inet_net_pton would always return 4 (bits)
for multicast addresses no matter how many octets were specified.
negotiated with Paul Vixie, original author of this function.
PR: standards/53151
Submitted by: Max Laier <max@love2party.net>
Optained from: OpenBSD