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, protocol 31 no longer sends it, while
older protocols are optimized so the sender just sends a valid rdev
value 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.