For some inputs with the AF_INET6 family, inet_cidr_ntop() may produce
invalid output like "::ffff:0.0", because it considers the prefix length
when determining how many octets of the IPv4 address to emit.

This behaviour doesn't make sense, because a mapped address always
contains a complete IPv4 address, which must be 4 octets; anything
else is an invalid address.

Fix this by removing the relevant code and simply emitting 4 octets
in all cases.

While here, add some tests for both inet_cidr_ntop and inet_cidr_pton
based on the inet_net_* tests.

MFC after: 1 week