(c-electric-delete, c-electric-delete-forward): Split `c-electric-delete'

(c-electric-delete, c-electric-delete-forward): Split `c-electric-delete'
into two functions where `c-electric-delete-forward' always deletes
forward and `c-electric-delete' only contains the code
necessary for XEmacs to choose between backward and forward
deletion.  `c-electric-delete-forward' is now bound to C-d to
get the electric behavior on that key too.

(c-fill-paragraph): Fixed bogus direct use of c-comment-prefix-regexp,
which caused an error when it's a list.