Added DEFGENERIC form for (SETF %SHEET-MEDIUM).

In order to get a clean build, I added this DEFGENERIC form, because
some Common Lisp compilers emit a warning when there is no explicit
DEFGENERIC form.

Also improved layout by removing parenthesis preceded by whitespace.
Added copyright and email address.
Added DEFGENERIC form for PORT-LOOKUP-SHEET.
Added DEFGENERIC form for PORT-UNREGISTER-MIRROR.
Updated copyright and email address.