Change backend-side COPY to write files with permissions 644 not 666

Change backend-side COPY to write files with permissions 644 not 666
(whoever thought world-writable files were a good default????).  Modify
the pg_pwd code so that pg_pwd is created with 600 permissions.  Modify
initdb so that permissions on a pre-existing PGDATA directory are not
blindly accepted: if the dir is already there, it does chmod go-rwx
to be sure that the permissions are OK and the dir actually is owned
by postgres.