U Area Removal

[David Schultz]

Over the years, the amount of data we have stored in each process' U
area has eroded to the point where all we have left are the following:

  - A struct kinfo_proc that is only used for a.out core dumps.
    This can be reconstructed at the time of the core dump, so
    it doesn't need to be there.

  - The struct pstats for the process, which takes a mere 216 bytes
    on i386.

In exchange for the ability to swap out this 216-byte structure, we
keep around a 4096-byte page, a 132-byte vm_object, and a couple of
pointers.  Moreover, there is a small amount of runtime overhead
associated with this, and developers need to remember to PHOLD() and
PRELE() the process as appropriate.[1]

I propose to remove the ability to swap the U area, allocating p_stats
from malloced memory instead.  Medium-term scheduling and swapping of
kernel stacks would be retained.  Here are the patches; !i386 testers
wanted:

	http://www.freebsd.org/~das/patches/upages.diff


[1] Most of the instances of PHOLD() and PRELE() right now never
    needed to be there or have been unnecessary ever since the PCB
    was moved out of the U area.