bug#24603: [RFC 07/18] Split up casify_region function.

[Eli Zaretskii]

> From: Michal Nazarewicz <mina86@mina86.com>
> Date: Tue,  4 Oct 2016 03:10:30 +0200
> 
> No functional changes at this time but splitting casify_region into
> a function dealing with multibyte and another dealing with unibyte
> buffers will make future code changes slightly easier.
> 
> * src/casefiddle.c (casify_region): Move most of the code into two
> new functions:
> (do_casify_multibyte_region, do_casify_unibyte_region): new functions.

Please make sure the multibyte version works with embedded eight-bit
characters, in both representations.  Some of the code you removed
could handle those cases.

Thanks.