chrony: does not support refclock section, nor specifying subnets explicitly

The chrony package has no support for 'refclock' settings. This is useful to have if you have say a GPS and/or PPS source connected to your openwrt device to discipline its clock precisely.

Also if you want to specify the subnet to serve NTP to, currently this can only be done on a per-interface basis. However it's cumbersome to create interfaces if your target subnet is larger than any individual interface (in the case i have, where i have a "dumb AP" device that serves NTP for multiple vlans (that i don't have interfaces for on this device since it's a dumb AP). It would be nice if chrony config also supported the ability to explicitly just list the subnets explicitly to allow queries from (without needing to jump through hoops adding them to mock interfaces which are then unrelated).

Will create a PR shortly to address these two shortcomings.

No files or tests are named. Start by locating the chrony package configuration and its current interface-based NTP access settings; then trace how package options are translated into chrony configuration. Done means supporting refclock settings and explicitly listed subnets, with coverage for both behaviors.