Detect when one tries to bind a lfun to a prefix of longer commands

Detect when one tries to bind a lfun to a prefix of longer commands

Change KeyMap::getBinding to return FuncRequest::prefix in this case.

Add handling of this case in PrefShortcuts::validateNewShortcut.

What does not seem to work is that if, for example, accent-acute is
bound to M-s (which is a prefix for size-related bindings), and then
the binding is removed, the functions are not visibly restored.

Part of bug #10131.