fold: move multi-byte character reading to a module

fold: move multi-byte character reading to a module

* gl/modules/mbbuf: New file.
* gl/lib/mbbuf.c: Likewise.
* gl/lib/mbbuf.h: Likewise.
* gl/local.mk (EXTRA_DIST): Add the new files.
* bootstrap.conf (gnulib_modules): Add mbbuf.
* src/fold.c: Include mbbuf.h.
(fold_file): Use the mbbuf functions instead of calling fread and
handling the input buffer ourselves.
* cfg.mk (exclude_file_name_regexp--sc_preprocessor_indentation)
(exclude_file_name_regexp--sc_GPL_version): Match gl/lib/mbbuf.c and
gl/lib/mbbuf.h.