Set the multibyte flag of the current buffer to FLAG.
If FLAG is t, this makes the buffer a multibyte buffer.
If FLAG is nil, this makes the buffer a single-byte buffer.
In these cases, the buffer contents remain unchanged as a sequence of
bytes but the contents viewed as characters do change.
If FLAG is `to', this makes the buffer a multibyte buffer by changing
all eight-bit bytes to eight-bit characters.
If the multibyte flag was really changed, undo information of the
current buffer is cleared. (fn FLAG)