for users who have customized it to a long list now redundant with
`mh-invisible-header-fields-default'.
(mh-invisible-header-fields-default-override): New defcustom.
- Users check off the fields they want displyed from what we
+ Users check off the fields they want displayed from what we
included in `mh-invisible-header-fields-default'.
(mh-invisible-headers): Function adapted to new variables.