(hexl-insert-multibyte-char) New function.

(hexl-insert-multibyte-char) New function.
(hexl-quoted-insert, hexl-self-insert-command)
(hexl-insert-hex-char, hexl-insert-decimal-char)
(hexl-insert-octal-char): Call it instead of hexl-insert-char.
Fix the doc strings accordingly.
(hexl-insert-char): Reject characters whose code is above 255.
Doc fix.
(hexl-mode-map): Copy the global keymap instead of creating a
sparse keymap, and bind all self-inserting characters to
hexl-self-insert-command.