Starting the conversion:
	* internal dev_t made 32bit.
	* new helpers - new_encode_dev(), new_decode_dev(), huge_encode_dev(),
huge_decode_dev(), new_valid_dev().  They do encoding/decoding of 32bit and
64bit values; for now huge_... are aliases for new_... and new_valid_dev()
is always true.  We do 12:20 for 32bit; representation is compatible with
16bit one - we have major in bits 19--8 and minor in 31--20,7--0.  That's
what the userland sees; internally we have (major << 20)|minor, of course.
	* MKDEV(), MAJOR() and MINOR() updated.
	* several places used to handle Missed'em'V dev_t (14:18 split)
manually; that stuff had been taken into common helpers.
	Now we can start replacing old_... with new_... and huge_..., depending
on the width available.  MKDEV() callers should (for now) make sure that major
and minor are within 12:20.  That's what the next chunk will do.