interrupt: add polarity & trigger parameters to unmask_irq

The XT-PIC obviously doesn't support different polarities, so that parameter
is ignored by pic_unmask_irq().
kip: add SchedulePrecision parameter for make_kip()
x86: optimize x86_get_eflags()

Sigh.
test: sched: add tests for ReadPrecision, SchedulePrecision

The latter has a few more rather large TODOs that'll be done eventually.
test: sched: improve the SchedulePrecision test a bit

Lots to do, still, but this one is at least good for detecting
SchedulePrecision error from 300 µs and up; the other one wasn't.