detect/iponly: fix netmask handling

detect/iponly: fix netmask handling

If the ipaddress was not the address range start, it was not masked to turn
it into that. So 1.2.3.4/24 was not stored as address 1.2.3.0 with netmask 24,
but as 1.2.3.4 with netmask 24. This was then propagated into the radix tree,
where it was used as an exact key in exact lookups, giving unexpected results.

This patch implements the netmask handling for IPv4 and IPv6, and adds a set
of tests for it.

Bug: #5081.
Bug: #5066.