eader_size > 4

Side note:  The lack of upwards compatibility sounds worse than it is.
All existing client implementations, libraw1394 and libdc1394, set
header_size = 4.  And since the ABI v1 behaviour does not offer any
advantages over the new behaviour, we deliberately do not provide the
old behaviour anymore.

Also add documentation about the format of fw_cdev_get_cycle_timer which
may be used in conjunction with the timestamp of iso packets but has a
different format.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
õûM