dhcp: T3316: Support hostname, DUID and MAC address in reservation

Reinstate support for hostname in DHCP reservation. Having hostname in
allows for server-side assignment of hostname. This is useful for static
lookup of hostname.

Ensure that hostname is a valid FQDN (doesn't have underscore, etc.)

Additionally, support using either of DUID or MAC address for
reservation. While MAC address is typically used for IPv4, and DUID is
typically used for IPv6, either of them can be used in IPv4 and IPv6
reservations in Kea.