DHCP: Enable OMAPI Config
There's currently no method to configure DHCP OMAPI settings other than modifying dhcpd.conf directly.
Fix #7304: Add OMAPI to the DHCP Server settings
Supports setting a port and setting or generating a key. Service is
enabled when settings are defined. Validation logic added on save to
verify port is valid and not in use as well as that a key is defined or
being generated. If validation fails, used is presented with descriptive
Also updated the write_settings call on the DHCP server page. No
arguments were being passed so changes came up as unkown on config
history screen. Config history will now reflect UI section and interface
where change was made.
Obtained from: https://github.com/pfsense/pfsense/pull/4077