The standard devices are currently broken since they have
the size ~0. As now files use loff_t as file size which is a signed
type the read implementation gets confused and now returns -1.

The current implementation also has the (somewhat theorical) problem
that we do not have real streaming devices, so /dev/zero went out
of zeroes after reading 4GB (or now LLONG_MAX).

This patch introduces a new cdev flag DEVFS_IS_CHARACTER_DEV and a new
file size flag FILE_SIZE_STREAM which makes it possible to create
real stream devices instead.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>