Require 250us lead time to start a vtimer.

Matches tests, improves the threads/vtimers/delete test.
Put a 250us minimum on all vtimer scheduling.
Simplify alarm timing.

This is from when it seemed like sceKernelSetAlarm() was by cycles.
Corrects handling of large timer / sysclock params.
Protect against vtimers scheduled in past.
Improve some timing in msgpipes.

Probably not super important, but makes tests happier.

Also, when unscheduling an event, return the current time left, including
already spent time since last Advance.
Adjust semaphore timing slightly.

This matches tests a bit better.
Eat some cycles when canceling/deleting fpls.

Improves tests measuing timeouts remaining after delete.
Eat some cycles when starting a thread.

Seems to improve a test a bit, based on tests.
Eat some cycles when signaling semaphores.

Seems to match tests.
Merge pull request #6822 from unknownbrackets/thread-funcs

Improve some timing / thread funcs