Get rid of recursion-marker values in enum AlterTableType

Get rid of recursion-marker values in enum AlterTableType

During ALTER TABLE execution, when prep-time handling of subcommands of
certain types determine that execution-time handling requires recursion,
they signal this by changing the subcommand type to a special value.
This can be done in a simpler way by using a separate flag introduced by
commit ec0925c22a3d, so do that.

Catversion bumped.  It's not clear to me that ALTER TABLE subcommands
are stored anywhere in catalogs (CREATE FUNCTION rejects it in BEGIN
ATOMIC function bodies), but we do have both write and read support for
them, so be safe.

Discussion: https://postgr.es/m/20220929090033.zxuaezcdwh2fgfjb@alvherre.pgsql