Use consistent type for pgaio_io_get_id() result

Mop-up for Datum conversion cleanups.

Remove useless/superfluous Datum conversions

Add missing Datum conversions

Fix incorrect lack of Datum conversion in _int_matchsel()

Suppress maybe-uninitialized warning.

Convert varatt.h access macros to static inline functions.

Fix varatt versus Datum type confusions

Improve prep_buildtree