[gfortran] patch for pr15472 and sequential unformatted I/O

[Paul Brook]

> the previous patch, plus your code (above), plus one missing semi-colon,
> passes all tests.
>
> ok to commit ?

Uh, sorry about this, but I've spotted something else :(

  int
  is_seekable (stream * s)
  {
!   /* by convention, if file_length == -1, the file is not seekable
!      note that a mmapped file is always seekable, an fd_ file may
!      or may not be. */

I'm not sure this is true, despite what the existing comment says. Both 
fd_flush and fd_truncate set s->file_length.
Maybe this doesn't matter, in which case we should add a comment saying why.

Paul