Handling a full range of keys for the menu key is not as trivial in FLTK as
with raw X11, so do what the Windows client did and restrict the available
keys to just the function keys.
git-svn-id: svn://svn.code.sf.net/p/tigervnc/code/trunk@4444 3789f03b-4d11-0410-bbf8-ca57d06f2519