XC-10218: Remove SELinux policy load and relabel operations.

The full policy load operation should never have been done in the
first place. The relabel operations are now down on firstboot.
XC-10218 Use if/then/fi instead of test && { block }.
XC-10218 Mount tmpfs for /tmp in new dom0 with proper label.

This should obviate the need for any hacky policy stuffs for
3.2.3 -> 4.0 OTA upgrades.