preserve the ABI and API for applications.  It was removed in the port
to amd64, but was remained as garbage giving a micro-pessimization and
spurious single-step traps on i386.

pcb_psl was intended to be used just to do a context switch of PSL_I,
but this context switch was null in most or all versions, and
mis-switching of PSL_T was done instead.

Some history:
- in 386BSD-0.0, cpu_switch() ran at splhigh() and splhigh() did too
  much interrupt disabling, so interrupts were hard-disabled across
  cpu_switch() and too many other places
- in 386BSD-0.0-patchkit through FreeBSD-4 and FreeBSD-5 before
  SMPng, splhigh() did soft interrupt masking, and cpu_switch() was
  excessively cautious and did a cli at the start and a sti at the
  end to hard-disable interrupts across the switch
- SMPng replaced the spl's and cli's by spinlocks (just sched_lock?),
  so interrupts were hard-disabled across cpu_switch() and too many
  other places again
- initial attempts to fix this intended to restore some soft
  interrupt disabling, but to support variations in this cpu_switch()
  used pushfl/popfl into pcb_psl to avoid hard-coding the assumption
  that the initial and final states have PSL_I enabled.  But the
  version with soft interrupt disabling wasn't used for long, or was
  never committed, (except I always used my different version of it
  for UP) so the pushfl/popl and pcb_psl to hold them have been doing
  less than nothing for about 14 years.