networkd: Add support for setting a preferred subnet id for IPv6 PD leases

This allows users to configure a subnet id that should be used instead
of automatically (sequentially) assigned subnets. The previous attempt
had the downside that the subnet id would not be the same between
networkd restarts. In some setups it is desirable to have predictable
subnet ids across restarts of services and systems.

The code for the assignment had to be broken up into two pieces. One of
them is the old (sequential) assignment of prefixes and the other is the
new assignment based on configured subnet ids. The new assignment code
has to be executed first and has to be taken into account when (later
on) allocating the "old" subnets from the same pool.

Instead of having one iteration through the links we are now trying to
allocate a prefix for every link on every delegated prefix, unless they
received an assignment in a previous iteration.