pgsql: Fix postgres --describe-config for guc enums, breakage noted by

Log Message:
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Fix postgres --describe-config for guc enums, breakage noted by Alvaro.

While at it, rename option lookup functions to make names clearer, per
discussion with Tom.

Modified Files:
--------------
    pgsql/src/backend/utils/misc:
        guc.c (r1.438 -> r1.439)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c?r1=1.438&r2=1.439)
        help_config.c (r1.20 -> r1.21)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/help_config.c?r1=1.20&r2=1.21)
    pgsql/src/include/utils:
        guc_tables.h (r1.40 -> r1.41)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/guc_tables.h?r1=1.40&r2=1.41)