Split "return to userspace via iretq" from intr_fast_exit into intr_user_exit.

Move AST handling from the bottom of alltraps and Xdoreti to the
 top of the new routine.
syscall-return-via-iretq and the FPU #DNA trap jump into intr_user_exit after
 the AST check (already performed for the former, skipped for the latter)
Delete a couple debugging hooks mlarkin@ and I used during Meltdown work

tested by many in snaps; thanks to brynet@ for spurious interrrupt testing
earlier reviews and comments kettenis@ mlarkin@; prodding from deraadt@

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