gjb writes: when doing encrypted zfs install, loader.conf should be updated
so bootpool is imported after boot:
        zpool_cache_load="YES"
        zpool_cache_type="/boot/zfs/zpool.cache"
        zpool_cache_name="/boot/zfs/zpool.cache"
otherwise /boot is a broken symlink after the system is up.
http://lists.freebsd.org/pipermail/freebsd-fs/2013-July/017891.html
NOTE: Fix a comment while here.

Reviewed by:	gjb
Document BSDINSTALL_TMPBOOT environment variable introduced by SVN r256343.
Move the PMC handling to the first level interrupt handler where it belongs.
Also add the pmc_hook use, to handle callchain tracing.

MFC after:	1 week
Add fasttrap for PowerPC.  This is the last piece of the dtrace/ppc puzzle.
It's incomplete, it doesn't contain full instruction emulation, but it should be
sufficient for most cases.

MFC after:	1 month
Fix SVN r256540: s/TMPETC/TMPBOOT/ for loader.conf(5) GELI entries.