pgsql: Add an 'enable_material' GUC.

Log Message:
-----------
Add an 'enable_material' GUC.

The logic for determining whether to materialize has been significantly
overhauled for 9.0. In case there should be any doubt about whether
materialization is a win in any particular case, this should provide a
convenient way of seeing what happens without it; but even with enable_material
turned off, we still materialize in cases where it is required for
correctness.

Thanks to Tom Lane for the review.

Modified Files:
--------------
pgsql/doc/src/sgml:
config.sgml (r1.267 -> r1.268)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml?r1=1.267&r2=1.268)
pgsql/src/backend/optimizer/path:
costsize.c (r1.216 -> r1.217)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/costsize.c?r1=1.216&r2=1.217)
joinpath.c (r1.132 -> r1.133)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/joinpath.c?r1=1.132&r2=1.133)
pgsql/src/backend/optimizer/plan:
subselect.c (r1.161 -> r1.162)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/plan/subselect.c?r1=1.161&r2=1.162)
pgsql/src/backend/utils/misc:
guc.c (r1.547 -> r1.548)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c?r1=1.547&r2=1.548)
postgresql.conf.sample (r1.282 -> r1.283)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/postgresql.conf.sample?r1=1.282&r2=1.283)
pgsql/src/include/optimizer:
cost.h (r1.100 -> r1.101)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/optimizer/cost.h?r1=1.100&r2=1.101)
pgsql/src/test/regress/expected:
rangefuncs.out (r1.24 -> r1.25)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/rangefuncs.out?r1=1.24&r2=1.25)