i386 4/4 change

Hi,
the change to provide full 4G of address space for both kernel and
user on i386 is ready to land.  The motivation for the work was to both
mitigate Meltdown on i386, and to give more breazing space for still
used 32bit architecture.  The patch was tested by Peter Holm, and I am
satisfied with the code.

If you use i386 with HEAD, I recommend you to apply the patch from
https://reviews.freebsd.org/D14633
and report any regressions before the commit, not after.  Unless
a significant issue is reported, I plan to commit the change somewhere
at Wed/Thu next week.

Also I welcome patch comments and reviews.