pgsql: Fix define_custom_variable so that SUSET custom variables behave

Log Message:
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Fix define_custom_variable so that SUSET custom variables behave
somewhat reasonably. It's not perfect, but it beats the kluge
proposed in the auto-explain patch ...

Modified Files:
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pgsql/src/backend/utils/misc:
guc.c (r1.478 -> r1.479)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c?r1=1.478&r2=1.479)