Remove SELinux policy load and relabel operations.

The full policy load operation should never have been done in the
installer to begin with. The relabel operations are now done on
firstboot.
Use if/then/fi instead of test && { block }.

This sort of flow control causes all sorts of problems. Specifically,
if placed at the end of a function the return value from said function
will indicate an error condition if the test fails.
Mount tmpfs on /tmp in new dom0 with proper label.

This should obviate the need for any hacky policy stuffs for OTA
upgrades ... at least for now.