PAE on i386

[Konstantin Belousov]

Hello,
at https://reviews.freebsd.org/D18894 I put a review which main goal is
to allow i386 kernels to use NX bits on capable hardware.  In essence,
single kernel now can operate using either PAE or non-PAE pagetables,
the selection is done at the cold (very early boot, before paging is
turned on) time.

This together with earlier 4/4 work gives much more life into i386 kernels
for whoever still needs them.  Please review/test, see the differential
review text for more explanation.