Change sending/receiving/storing of the rdev value for special files.

Change sending/receiving/storing of the rdev value for special files.
Since the value is not needed, the (superfluous) sending of the value
is optimized so that a valid rdev value is sent as efficiently as
possible.  The receiver no longer caches an rdev value for special
files, and the generator will always pass a 0 rdev value to do_mknod()
for special files. Fixes bug #6280.