Follow cl-xkbcommon 767be00: modifier keywords as a type
A keyword that names no modifier is now a type-error, since the
functions that take a modifier declare the new type modifier-keyword.
The reference describes the type, Concepts names it beside the
keywords, and Errors and Logging and Keyboard States and Modifiers say
which error a wrong keyword and a wrong name signal, quoting SBCL's
message for the keyword.
