This adds a new `Hostname=` option to the [DHCPServerStaticLease]
section in .network files, allowing an administrator to assign a
specific hostname to a client receiving a static lease.
We automatically select the correct DHCP option to use based on the
format of the provided string:
- Single DNS labels are sent as Option 12.
- Names with multiple DNS labels are sent as Option 81 in wire format.
Fixes: #39634
7f9c0c31d2