MP Implmentation 3/4: MAJOR progress on SMP, full userland MP is now working!

MP Implmentation 3/4: MAJOR progress on SMP, full userland MP is now working!
A number of issues relating to MP lock operation have been fixed, primarily
that we have to read %cr2 before get_mplock() since get_mplock() may switch
away.  Idlethreads can now safely HLT without any performance detriment.
The userland scheduler has been almost completely rewritten and is now
using an extremely flexible abstraction with a lot of room to grow.  pgeflag
has been removed from mapdev (without per-page invalidation it isn't safe
to use PG_G even on UP).  Necessary locked bus cycles have been added for
the pmap->pm_active field in swtch.s.  CR3 has been unoptimized for the
moment (see comment in swtch.s).  Since the switch code runs without the
MP lock we have to adjust pm_active PRIOR to loading %cr3.
Additional sanity checks have been added to the code (see PARANOID_INVLTLB
and ONLY_ONE_USER_CPU in the code), plus many more in kern_switch.c.
A passive release mechanism has been implemented to optimize P_CURPROC/lwkt
priority shifting when going from user->kernel and kernel->user.
Note: preemptive interrupts don't care due to the way preemption works so
no additional complexity there.  non-locking atomic functions to protect
only against local interrupts have been added.  astpending now uses
non-locking atomic functions to set and clear bits.  private_tss has been
moved to a per-cpu variable.   The LWKT thread module has been considerably
enhanced and cleaned up, including some fixes to handle MPLOCKED vs td_mpcount
races (so eventually we can do MP locking without a pushfl/cli/popfl combo).
stopevent() needs critical section protection, maybe.