use file_ops and get_file_from_fd() for console

use file_ops and get_file_from_fd() for console

Add struct file_ops for the console related file types (FTYPE_CONSOLE
and savefile). FTYPE_CONSOLE remains statically defined, as it is used
to statically init stdin, stdout and stderr.

Instead of directly accessing the files[] array use get_file_from_fd().

With CONSOLE now handled via file_ops the bogus file descriptor case in
select_poll() now needs to be handled more explicit instead of dropping
into console handling, assuming that this case was basically meant to
cover SAVEFILE.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>