pgsql: Make autovacuum_enabled a ternary reloption.

Make autovacuum_enabled a ternary reloption.

This commit reimplements autovacuum_enabled as a ternary, using the
support added in commit 4d6a66f675 and following the example of
vacuum_truncate. This changes only the internal representation: an
unset value still behaves as enabled, 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: Greg Burd <greg(at)burd(dot)me>
Reviewed-by: Sami Imseih <samimseih(at)gmail(dot)com>
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/3d8f71a370f0ad93762f140f6e69c2abe05b229a

Modified Files
--------------
src/backend/access/common/reloptions.c | 19 +++++++++----------
src/backend/catalog/index.c | 3 ++-
src/backend/postmaster/autovacuum.c | 2 +-
src/include/utils/rel.h | 2 +-
4 files changed, 13 insertions(+), 13 deletions(-)