(byte-compile-insert-header): Do not look for multibyte characters here.

(byte-compile-insert-header): Do not look for multibyte characters here.
Generate the right file header for use if there are none.
Insert a line of semicolons for subsequent deletion if needed.
(byte-compile-fix-header): New function.
This updates the file header if the file uses multibyte characters.
(byte-compile-from-buffer): Call byte-compile-insert-header
before compiling, and byte-compile-fix-header at the end.