Simple wrapper for make_unibyte_string, adjust font_open_by_name.

Simple wrapper for make_unibyte_string, adjust font_open_by_name.
* src/lisp.h (build_unibyte_string): New function.
* src/dosfns.c, src/fileio.c, src/fns.c, src/ftfont.c, src/process.c:
* src/sysdep.c, src/w32fns.c, src/xfns.c: Use it.
* src/font.c (font_open_by_name): Change 2nd and 3rd args to the only arg
of type Lisp_Object to avoid redundant calls to make_unibyte_string.
Adjust users accordingly.
* src/font.h (font_open_by_name): Adjust prototype.
* admin/coccinelle/unibyte_string.cocci: Semantic patch to convert from
make_unibyte_string to build_unibyte_string where appropriate.