Make State button interaction less confusing

Make State button interaction less confusing

* lisp/cus-edit.el (custom-variable-current-value): New function.
(custom-variable-backup-value): Use it.
(custom-variable-set, custom-variable-mark-to-reset-standard): Check
that old value is different than the new one.  If it is, make a
backup.  This way, we avoid offering the Set to Backup Value
unnecesarily.
(custom-variable-reset-saved): Reset the variable-comment property for
the variable, to help custom-variable-state be more correct.  Also
check if we should backup old value.
(custom-variable-state): If a variable was set to the standard value,
say its state is standard rather than set, which is more correct.
Getting the right variable state is important for menu options to be
enabled/disabled, and for displaying the right message to the user
(bug#12864).