Allow to unbind without specifying the lfun

Allow to unbind without specifying the lfun

When unbinding a shortcut, it may happen that the exact definition of
the request is not known. A typical example it Tab, which is bound to
a complex command sequence.

In this case it is convenient to use the syntax
\unbind "Tab" "*"

To make this word, the special "*" value is translated to the
FuncRequest::unknown lfun and this value is considered specially in
several places.