Only regenerate atomic.S if atomic.c changed.
Fix 'make install'.
Adjust physmem by PAGESIZE, as ZOL expects.
Rip out MNTTAB, fix zfs autoload/detection, etc...
Hack gethrtime() to work in userspace.

In the future, lib/libspl should be pushed to the spl repo, so we can reduce
code duplication.  zfs_abs_to_nano is not available here...
Add cmd.sh, a generic userspace command wrapper.