Add ternary reloption type

There is a tendency for boolean reloptions in PostgreSQL code, one with "on"
and "off" values, to be replaced with options with "on", "off",
"[use_global_settings]" behaviour.

For `vacuum_index_cleanup" and gist's `buffering` reloption such behavior have
been implemented as enum-tyoe option.

For vacuum_truncate this behaviour have been umplemented by adding additional
`is_set` flag to `bytea` representation of the reloptions.

Both solutions looks like workaround hacks to implement option with three
available states.

This patch introduce "ternary" reloption type, that behave like bool option,
but also has an additional "unset" state. This state may be reachable only by
not setting or RESETting an option, like in `vacuum_truncate` option, or it
may have some text alias that allow user to explicitly set it, like "auto" in
`vacuum_index_cleanup` or gist's `buffering`

`vacuum_truncate`, `vacuum_index_cleanup` and gist's `buffering` reloptions are
reimplemented as ternary  reloptions without significant behaviour changes.

Author: nataraj-hates-MS-for-stealing-github

src/test/regress/expected/reloptions.out

@@ -98,6 +98,42 @@ SELECT reloptions FROM pg_class WHERE oid = 'reloptions_test'::regclass;
  {fillfactor=13,autovacuum_enabled=false}
 (1 row)
 
+-- Tests for future (FIXME) ternary options
+-- behave as boolean option: accept unassigned name and truncated value
+DROP TABLE reloptions_test;
+CREATE TABLE reloptions_test(i INT) WITH (vacuum_truncate);
+SELECT reloptions FROM pg_class WHERE oid = 'reloptions_test'::regclass;
+       reloptions       
+------------------------
+ {vacuum_truncate=true}
+(1 row)
+
+DROP TABLE reloptions_test;
+CREATE TABLE reloptions_test(i INT) WITH (vacuum_truncate=FaLS);
+SELECT reloptions FROM pg_class WHERE oid = 'reloptions_test'::regclass;
+       reloptions       
+------------------------
+ {vacuum_truncate=fals}
+(1 row)
+
+-- preferred "true" alias is used when storing
+DROP TABLE reloptions_test;
+CREATE TABLE reloptions_test(i INT) WITH (vacuum_index_cleanup=on);
+SELECT reloptions FROM pg_class WHERE oid = 'reloptions_test'::regclass;
+        reloptions         
+---------------------------
+ {vacuum_index_cleanup=on}
+(1 row)
+
+-- custom "third" value is available
+DROP TABLE reloptions_test;
+CREATE TABLE reloptions_test(i INT) WITH (vacuum_index_cleanup=auto);
+SELECT reloptions FROM pg_class WHERE oid = 'reloptions_test'::regclass;
+         reloptions          
+-----------------------------
+ {vacuum_index_cleanup=auto}
+(1 row)
+
 -- Test vacuum_truncate option
 DROP TABLE reloptions_test;
 CREATE TEMP TABLE reloptions_test(i INT NOT NULL, j text)

src/test/regress/sql/reloptions.sql

@@ -59,6 +59,27 @@ UPDATE pg_class
 ALTER TABLE reloptions_test RESET (illegal_option);
 SELECT reloptions FROM pg_class WHERE oid = 'reloptions_test'::regclass;
 
+-- Tests for future (FIXME) ternary options
+
+-- behave as boolean option: accept unassigned name and truncated value
+DROP TABLE reloptions_test;
+CREATE TABLE reloptions_test(i INT) WITH (vacuum_truncate);
+SELECT reloptions FROM pg_class WHERE oid = 'reloptions_test'::regclass;
+
+DROP TABLE reloptions_test;
+CREATE TABLE reloptions_test(i INT) WITH (vacuum_truncate=FaLS);
+SELECT reloptions FROM pg_class WHERE oid = 'reloptions_test'::regclass;
+
+-- preferred "true" alias is used when storing
+DROP TABLE reloptions_test;
+CREATE TABLE reloptions_test(i INT) WITH (vacuum_index_cleanup=on);
+SELECT reloptions FROM pg_class WHERE oid = 'reloptions_test'::regclass;
+
+-- custom "third" value is available
+DROP TABLE reloptions_test;
+CREATE TABLE reloptions_test(i INT) WITH (vacuum_index_cleanup=auto);
+SELECT reloptions FROM pg_class WHERE oid = 'reloptions_test'::regclass;
+
 -- Test vacuum_truncate option
 DROP TABLE reloptions_test;