Handle page faults within the virtual kernel process itself (what would be

Handle page faults within the virtual kernel process itself (what would be
called kernel mode page faults in a real kernel).  Fortunately the system
trapframe is a subset of the ucontext supplied to userland signal function.

Since user vs kernel addresses and trap types cannot be discerned by looking
at the frame (because they are in entirely different VM spaces), separate
trap() into user_trap() and kern_trap().

Implement a poor-man's (really kludgy) version of copyin, copyout, and related
functions.

Implement and process T_PAGEFLT virtual kernel traps.  They work about 85% of
the time.

UFS is now able to mount the root filesystem.