Support DNS name in Client Identifier

The Client Identifier setting (when defining a New Client) should also support DNS names, e.g. `phone.local` in addition to IP addresses. The names can be resolved and turned into IP or MAC addresses internally. Useful for using more user friendly names. This requires that local DNS works, which may not always be the case, but for those who know how to manage it's a nice feature.

Or allow for some syntax to add a comment to go with the IP/MAC so we can easily remember which device that is. This is for the case when multiple IPs are used to identify a group of clients.

Note that I do not use the built-in DHCP server because it doesn't support multiple VLANs (see #1181), but I do have an upstream configured that can resolve these names (local dnsmasq that also does multi-VLAN DHCP).