* ew-dec.el (ew-decode-us-ascii): New function.

* ew-dec.el (ew-decode-us-ascii): New function.
(ew-decode-none): Use the function `ew-decode-us-ascii' instead of
the variable `ew-decode-us-ascii'.
(ew-decode-generic): Ditto.
(ew-decode-generic-others): Ditto.

* ew-unit.el (ew-charset-aliases): Abolished.
(ew-char-decoder): Use `decode-mime-charset-string' with 'LF as lbt.

* ew-var.el (ew-decode-us-ascii): Abolished.
(ew-default-mime-charset): New variable.
(ew-dynamic-options): Add `ew-default-mime-charset'.