[plt-scheme] current-alist-separator-mode

I have changed the defaul behavior of `current-alist-separator-mode'
to use ampersands instead of semicolons.  This is for the same
(default) value of 'amp-or-semi.  Some code will probably use `&'s
now, if you didn't set an explicit behavior.

I also added a new option for this parameter -- 'semi-or-amp -- which
is like the old default (parse both, generate semicolon separators).

This change going to be included in 371, to be out soon.

-- 
          ((lambda (x) (x x)) (lambda (x) (x x)))          Eli Barzilay:
                  http://www.barzilay.org/                 Maze is Life!