Restore comment/string check for 'electric-layout-mode'

Restore comment/string check for 'electric-layout-mode'

This reverts an accidental change which allowed
'electric-layout-mode' to insert newlines inside strings and
comments.  The new behavior can be obtained by setting the
new variable 'electric-layout-allow-in-comment-or-string' to a
non-nil value.
* lisp/electric.el (electric-layout-allow-in-comment-or-string):
New variable to determine whether inserting newlines is
permitted within comments or strings.
(electric-layout-post-self-insert-function-1): Restore the
previous default behavior of not inserting newlines within
comments or strings.