[FEAT] Renice or set process priority

[rcarmo]

Is this a new feature request?

• I have searched the existing issues

Wanted change

Create an environment variable setting that sets the syncthing process nice level and/or priority.

Reason for change

I have a few machines where I can't really set docker CPUlimits due to lack of kernel features (Synology is a notorious offender) and it would be very nice to be able to set the nice level/priority through an environment variable - limiting indexers and hashers to 1 or 2 goes a long way, but even then reindexing large folders tends to completely swamp low end CPUs. Going into the container manually sort of works, but given that this container ships with a nice init-like system it would be great to have it take an environment variable and set the syncthing process priority somehow.

Proposed code change

No response

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