Restore shortcuts on help screens (Closes: #764154, #781853, #781992).

The following ones (at least) appear to work:
  install, installspk, expert, menu

That one doesn't, probably because it's only a way to get back from a
submenu. This shouldn't be too much of an issue though:
  mainmenu

Probably an undesired change in the following commit:
| commit 1e996833
| Author: Joey Hess <joey@kitenet.net>
| Date:   Mon Sep 22 14:36:51 2014 -0400
|
|     Remove desktop selection from syslinux; now available in tasksel.
|
|     This will need some follow-on changes in debian-cd.
|
|     Also, removed forcing of xfce default for win32-loader, except kept it for
|     kfreebsd. win32-loader also needs to remove its desktop menu (bug filed).

With many thanks to Vaclav Ovsik for the pointer.