In pg_upgrade, copy fsm, vm, and extent files by checking for file

In pg_upgrade, copy fsm, vm, and extent files by checking for file
existence via open(), rather than collecting a directory listing and
looking up matching relfilenode files with sequential scans of the
array.  This speeds up pg_upgrade by 2x for a large number of tables,
e.g. 16k.

Per observation by Ants Aasma.