Make pg_stat_file() use OUT parameters so that the user doesn't have to

Make pg_stat_file() use OUT parameters so that the user doesn't have to
remember the output parameter set for himself.  It's a bit of a kluge
but fixing array_in to work in bootstrap mode looks worse.
I removed the separate pg_file_length() function, as it no longer has any
real notational advantage --- you can write (pg_stat_file(...)).length.