Simplify str_to_multibyte and related code

Simplify str_to_multibyte and related code

* src/character.h (str_to_multibyte):
* src/character.c (str_to_multibyte): Remove `nbytes` argument; return
it instead.  Copy forwards.
* src/fns.c (concat_to_string, Fstring_make_multibyte):
Use str_to_multibyte.
(string_make_multibyte): Remove.
(string_to_multibyte):
* src/print.c (print_string):  Adapt calls.