pg_upgrade: adjust umask() calls

pg_upgrade:  adjust umask() calls
Since pg_upgrade -j on Windows uses threads, calling umask()
before/after opening a file via fopen_priv() is no longer possible, so
set umask() as we enter the thread-creating loop, and reset it on exit.
Also adjust internal fopen_priv() calls to just use fopen().
Backpatch to 9.3beta.