pgsql: Fix pgxs.mk to always add --dbname=$(CONTRIB_TESTDB) to REGRESS

Fix pgxs.mk to always add --dbname=$(CONTRIB_TESTDB) to REGRESS_OPTS.

The previous coding resulted in contrib modules unintentionally overriding
the use of CONTRIB_TESTDB. There seems no particularly good reason to
allow that (after all, the makefile can set CONTRIB_TESTDB if that's really
what it intends).

In passing, document REGRESS_OPTS where the other pgxs.mk options are
documented.

Back-patch to 9.1 --- in prior versions, there were no cases of contrib
modules setting REGRESS_OPTS without including the --dbname switch, so
while the coding was fragile there was no actual bug.

Branch
------
REL9_1_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/5354abb8f69734ff1c07cee5e29845b74d46e9d6

Modified Files
--------------
contrib/sepgsql/Makefile | 4 +++-
doc/src/sgml/extend.sgml | 9 +++++++++
src/makefiles/pgxs.mk | 7 +++----
3 files changed, 15 insertions(+), 5 deletions(-)