sysmon(9): New sysmon_task_queue_barrier(pri) function.

This waits for the completion of all tasks at priority pri or lower
that are currently queued at the time of the call.

Replace a home-grown run-once implementation with the real RUN_ONCE()

This really does need to return an error code, so change the
sysmon_task_queue_fini() function to have a non-void type.

Convert sysmon_taskqueue to use mutex(9) and condvar(9) and initialize
them in init_main.c via sysmon_task_queue_preinit().

Reviewed and ok by ad@.

Generic "task queue" for sysmon back-ends. This provides a thread
context for callbacks that require it in order to handle sysmon-related
events.

Adapted from the "OsdSchedule" bits of the ACPI cod

Generic "task queue" for sysmon back-ends. This provides a thread
context for callbacks that require it in order to handle sysmon-related
events.

Adapted from the "OsdSchedule" bits of the ACPI code.

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