pgsql: In pg_upgrade, copy fsm, vm, and extent files by checking for fi

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.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/29add0de4920e4f448a30bfc35798b939c211d97

Modified Files
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contrib/pg_upgrade/file.c        |   55 ----------
contrib/pg_upgrade/pg_upgrade.h  |    2 -
contrib/pg_upgrade/relfilenode.c |  208 +++++++++++++++-----------------------
3 files changed, 82 insertions(+), 183 deletions(-)