statime-linux: PHC-only mode

[teodly]

If:

• hardware timestamping is supported, and
• we don't want to synchronize CLOCK_TAI/REALTIME to PTP ( Use software timestamping with PHC and hardware timestamping available #380 , Independent clock for arbitrary timescale #389 ), and
• we are operating either as a PTP slave only, or in an arbitrary timescale, and
• statime daemon is using only one PHC

it doesn't make sense to run the clock_task. When other apps need the clock, they can open /dev/ptp and read it directly.

Could be useful for PipeWire AES67 and simpler and more precise than #389 .

This mode would be a configuration option.