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)