register trashed the low byte.
(set-register-value): Fixed test so the value 0 can be set.
(set-register-value): Rewrote to use bit operations instead of
multiplication and division.
(register-name-by-word-alist, register-name-by-byte-alist):
Combined into one list, register-name-alist.
(register-value, set-register-value): Use combined list.
(mode-line-format): Make the %n pure.
