Improvements and fixes for Advanced DHCP Options

[pmilazzo]

• The "Always" checkbox now works for options with an explicit tag, and is disabled for untagged options; the latter cannot be supported because of a limitation in the OpwnWRT configuration language.
• Tagging by host name has been removed; it didn't work before and isn't particularly useful in the AREDN context.
• Tagging by Agent Circuit ID, Agent Remote ID, and Subscriber-ID are now supported so that a DHCP Relay Agent can be used to extend the LAN across multiple subnets.
• Small improvements were made to error handling and hints.