Fix comparison of current values for the key-sequence :type

Fix comparison of current values for the key-sequence :type

* lisp/cus-edit.el (custom-variable-modified-p): Round-trip
the option value before comparing it against the widget's value.
This mostly fixes comparison against the obsolete key-sequence
widget, but could fix other corner cases, when the widget
accepts different types as values.  (Bug#76156)

* test/lisp/cus-edit-tests.el (cus-edit-test-bug76156)
(cus-edit-test-bug76156-2): New test options.
(cus-edit-test-unedited-option): New test.

(cherry picked from commit f549cedaa2721bfc463fe714c00016aed21f7b5c)