pmap_install() needs to be atomic WRT to context switching. Protect

pmap_install() needs to be atomic WRT to context switching. Protect
switching user regions (region 0-4) with schedlock. Avoid unnecessary
recursion on schedlock by moving the core functionality to another
function (pmap_switch()) where we assert schedlock is held. Turn
pmap_install() into a wrapper that grabs schedlock. This minimizes
the number of callsites that need to be changed.
Since we already have schedlock in cpu_switch() and cpu_throw(),
have them call pmap_switch() directly. These were also the only two
calls to pmap_install() outside pmap.c, so make pmap_install() static
and remove its prototype from pmap.h

Approved by: re (blanket)