9p: Missing call to reply_with_perror.
daemon: Add more information to certain calls to perror.

Replace selected calls to 'perror (filename)' with:

  fprintf (stderr, "syscall: %s: %m\n", filename);

so that more information is available about precisely which syscall
failed.

Note this is *not* reply_with_perror.  These messages are only printed
in verbose output, for the benefit of debugging.
daemon: download: Add extra check that download file is not a directory (RHBZ#908321).