vmspace changes to use sysref

[Matthew Dillon <dillon@xxxxxxxxxxxxxxxxxxxx>]

    This was kinda a whopper of a commit to HEAD.  I changed struct vmspace
    over to using the new SYSREF API, but I had to fix a number of other
    issues above and beyond the basic changes which were revealed by the
    work.

    I'd appreciate it if a couple of people could help test these changes.
    SYSREF is going to be used all over the place and it has to be solid.
    vmspace was one of the more difficult structures to convert.

    Keep your eye out for VM and PMAP related panics, particulary panics
    like: 'vmfault: fault on nofault entry'.  My overnight test succeeded
    by I am not 100% sure I've found all the cases.  PMAP teardown races
    usually result in no debugger prompt after the panic, or a complete
    lockup.

						-Matt