SharedFileSet allows temporary files to be created by one backend and
then exported for read-only access by other backends, with clean-up
managed by reference counting associated with a DSM segment.  This
includes changes to fd.c and buffile.c to support the new kind of
temporary file.

This will be used by an upcoming patch adding support for parallel
hash joins.

Author: Thomas Munro
Reviewed-By: Peter Geoghegan, Andres Freund, Robert Haas, Rushabh Lathia
Discussion:
    https://postgr.es/m/CAEepm=2W=cOkiZxcg6qiFQP-dHUe09aqTrEMM7yJDrHMhDv_RA@mail.gmail.com
    https://postgr.es/m/CAH2-WznJ_UgLux=_jTgCQ4yFz0iBntudsNKa1we3kN1BAG=88w@mail.gmail.com