ps(1): reuse keyword "cpu" to show CPU number

This flag will now show the processor number on which a process is running.

This change was inspired by PR129965. Initially I didn't think that the
patch attached to it was correct -- it sacrificed ki_estcpu use in "cpu"
for ki_lastcpu and I thought that the old functionality should be kept and
the new (cpu#) one added to it. But I've since discovered that ki_estcpu is
sched_4bsd-specific. What's worse, it represents the same thing as
ki_pctcpu, except ki_pctcpu is universal -- so "%cpu" has been using it
successfully. Therefore, I've decided to replace information based on
ki_estcpu with information based on ki_oncpu/ki_lastcpu.

Key parts of the code and manual changes were borrowed from top(1).

PR: 129965
Reported by: Nikola Knežević
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D25377