First draft of advanced DHCP option specification on Ports tab.

[pmilazzo]

Description

Allows the node administrator to specify additional DHCP options that
will be supplied to LAN clients in specific circumstances. This change
adds two tables to the Ports configuration tab.

The "Tags for Advanced DHCP Options" table allows the administrator to
specify DHCP tags that will be assigned to clients that identify
themselves with specific values for properties such as Vendor Class or
MAC address.

The "Advanced DHCP Options" table allows the administrator to specify
arbitrary DHCP options to send to any client, or only to clients with a
specific tag. Option numbers can be entered directly or chosen from a
list of well-known options. Option values are manually entered by the
administrator.

In-browser validation is implemented for all input fields with easily
recognizable content such as host names, MAC addresses, and port and
option numbers. Placeholders are also supplied for input fields, such as
MAC addresses with wildcard matching, that might otherwise be difficult
to describe.

Issues with the current version:

• Sending DHCP options not requested by the client is implemented using
  the dhcp_option_force UCI configuration option, but does not currently
  work.
• Tagging by client host name is supported by dnsmasq, but not yet by
  UCI.
• DHCP option values must be entered manually by the administrator, but
  are not currently validated.