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.

contrib/sepgsql/Makefile

@@ -5,11 +5,9 @@ OBJS = hooks.o selinux.o uavc.o label.o dml.o \
 	database.o schema.o relation.o proc.o
 DATA_built = sepgsql.sql
 
-REGRESS = label dml misc
-REGRESS_PREP = check_selinux_environment
-REGRESS_OPTS = --launcher $(top_builddir)/contrib/sepgsql/launcher
-
-EXTRA_CLEAN = -r tmp *.pp sepgsql-regtest.if sepgsql-regtest.fc
+# Note: because we don't tell the Makefile there are any regression tests,
+# we have to clean those result files explicitly
+EXTRA_CLEAN = -r $(pg_regress_clean_files) tmp/ *.pp sepgsql-regtest.if sepgsql-regtest.fc
 
 ifdef USE_PGXS
 PG_CONFIG = pg_config
@@ -23,6 +21,3 @@ include $(top_srcdir)/contrib/contrib-global.mk
 endif
 
 SHLIB_LINK += -lselinux
-
-check_selinux_environment:
-	@$(top_builddir)/contrib/sepgsql/chkselinuxenv "$(bindir)" "$(datadir)"