Fix the userland scheduler. When the scheduler releases the P_CURPROC

Fix the userland scheduler.  When the scheduler releases the P_CURPROC
designation it unconditionally handed it off to the highest priority
process on the userland process queue, ignoring the fact that the 'current'
process might have had a higher priority.  There was also a missing call to
lwkt_maybe_switch() in the resched_wanted() case that could cause interrupt
threads to stall for a long period of time when they could not preempt.

In SMP there are still some issues.  Niced processes work better, but at
the moment the P_CURPROC handoff does not take into account the fact that
the new higher priority process might better be handed off to another cpu
that is running a lower priority process then the current cpu.