Add a 10 millisecond delay after sending the initial INIT IPI.  This
matches the algorithm in the MP specification (1.4).  Previously we
were sending out the deassert INIT IPI immediately after the initial
INIT IPI was sent.

(cherry picked from commit 6c62ea1c5136b13bea147d023af84b777d4219b0)
Remove the deassert INIT IPI from the IPI startup sequence for APs.
It is not listed in the boot sequence in the MP specification (1.4),
and it is explicitly ignored on modern CPUs.  It was only ever required
when bootstrapping systems with external APICs (that is, SMP machines
with 486s), which FreeBSD has never supported (and never will).

While here, tidy some comments and remove some banal ones.

(cherry picked from commit 7d55435a891039931540cc173244912a7acb20f3)