bug#4429: purecopy calls needed for :help and in menu-bar.el

[Dan Nicolaescu]

Juri Linkov <juri@jurta.org> writes:

  > >> Does anyone know a reason why one menu item in menu-bar.el
  > >> (`separator-exit') doesn't use the standard format with `menu-item'
  > >> like all other menu items use?
  > >
  > > Accident?
  > 
  > Probably.  Fixed.

I think that a better fix would be to purecopy '("--"), there are >60
instances of this in GC memory right when emacs starts up.

Maybe define a constant menu-bar-standard-separator and use that
everywhere?