Add 'trunc_to_net' flag to resolve_ipnets() function.

This new flag will cause resolve_ipnets() to mask the resolved IP
addresses with the specified netmask, thus "truncating" the address
down to the network address for the subnet.  For example, if the
hostname "exempel.se" resolves to 2001:db8:17:23::69:3:4711, then
"exempel.se/64" will return 2001:db8:17:23::/64, "exempel.se/112"
will return 2001:db8:17:23::69:3:0/112, and "exempel.se/96" will
yield the address 2001:db8:17:23:0:69::/96.

This can be useful if you have an "example" host, and want to specify
the entire subnet on which that host resides, but your application
requires the address part of the CIDR specification given to it to be
the network address, i.e. with all bits in the host part to be zero.