pgsql: Take sepgsql regression tests out of the regular regression test

Take sepgsql regression tests out of the regular regression test mechanism.

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.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/cc4ff8742b99d3b20a52f529d03bbe802f4b0053

Modified Files
--------------
contrib/sepgsql/Makefile | 11 +--
contrib/sepgsql/chkselinuxenv | 230 ----------------------------------
contrib/sepgsql/test_sepgsql | 264 +++++++++++++++++++++++++++++++++++++++
doc/src/sgml/sepgsql.sgml | 277 ++++++++++++++++++++++------------------
4 files changed, 419 insertions(+), 363 deletions(-)