Fix bug with string values in equal-including-properties

Fix bug with string values in equal-including-properties

* src/intervals.c (intervals_equal_1): Factor out from
intervals_equal.  Optionally use Fequal for comparison of string
values in property lists.
(intervals_equal): Update for the above.
(compare_string_intervals): Use the above optional Fequal comparison
to fix a bug where 'equal-including-properties' compared strings with
eq, instead of equal.  (Bug#6581)
* test/src/fns-tests.el (fns-tests-equal-including-properties)
(fns-tests-equal-including-properties/string-prop-vals): New tests.

* test/lisp/emacs-lisp/ert-tests.el
(ert-test-equal-including-properties): Remove parts testing
'equal-including-properties'.
* lisp/emacs-lisp/ert.el (ert-equal-including-properties): Add
FIXME that this should be removed.