experimental PMAP_DIRECT if available; the direct code paths now survive
longer than the pmap_enter() variant, but still triggers panic during
build.sh tools run; remove some obsolete sysctls
add some XXXs to mark places which need attention to make this more stable
Note: the loan case is now actually significantly slower than the
non-loan case on MP systems, due to synchronous IPIs triggered by
marking the page read-only by uvm_loan(); this is being discussed
in the email thread
https://mail-index.netbsd.org/tech-kern/2018/05/21/msg023441.html
that is basically the same issue due to which loaning was disabled
for sosend()
3d8c9e20d7