pgsql: Add an "unset" value for vacuum_index_cleanup.

Add an "unset" value for vacuum_index_cleanup.

This commit adds a new value to StdRdOptIndexCleanup to distinguish
whether it is explicitly set, similar to ViewOptCheckOption's
VIEW_OPTION_CHECK_OPTION_NOT_SET. This changes only the internal
representation; an unset value still defaults to AUTO, and the
option accepts the same input as before.

This is preparatory work for a follow-up commit that will make use
of the new "unset" state.

Reviewed-by: Michael Paquier <michael(at)paquier(dot)xyz>
Reviewed-by: Sami Imseih <samimseih(at)gmail(dot)com>
Tested-by: Greg Burd <greg(at)burd(dot)me>
Tested-by: solai v <solai(dot)cdac(at)gmail(dot)com>
Discussion: https://postgr.es/m/aFRxC1W_kZU9OjJ9%40nathan

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/c66e7531e4141d5fa4accaeafaec4e7c2f638da0

Modified Files
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src/backend/access/common/reloptions.c | 3 ++-
src/backend/commands/vacuum.c | 23 ++++++++++++++---------
src/include/utils/rel.h | 1 +
3 files changed, 17 insertions(+), 10 deletions(-)