Because these tests require root privileges, not to mention invasive
changes to the security configuration of the host system, it's not
reasonable for them to be invoked by a regular "make check" or "make
installcheck".  Instead, dike out the Makefile's knowledge of the tests,
and change chkselinuxenv (now renamed "test_sepgsql") into a script that
verifies the environment is workable and then runs the tests.  It's
expected that test_sepgsql will only be run manually.

While at it, do some cleanup in the error checking in the script, and
do some wordsmithing in the documentation.