(mime-iso646-character-unification-alist): Move to mcs-xmu.el.

(mime-iso646-character-unification-alist): Move to mcs-xmu.el.
(mime-unified-character-face): Move to mcs-xmu.el.
(mime-character-unification-limit-size): Move to mcs-xmu.el.
(decode-mime-charset-region-with-iso646-unification): Move to mcs-xmu.el.

(decode-mime-charset-region-with-iso646-unification): Narrow to the region
while decoding; bind `case-fold-search' to nil.
(mime-character-unification-limit-size): Make it can also be nil which means
the size is unlimited.

(decode-mime-charset-region-with-iso646-unification): Don't put
`mime-unified-character-face' to unified text if it is nil.
(mime-character-unification-limit-size): Fix doc string.