bug#27530: patch to cut and copy secondary

[Tak Kunihiro]

>> I often want to comment out text with mouse-secondary-overlay.
>> 
>> I meant if `mouse-secondary-overlay -> region' direction is not
>> supported, text with mouse-secondary-overlay cannot be commented out
>> until I select the text again as region.  I thought it is good to
>> avoid redundant operation if possible.
> 
> If we want a function that will put region around secondary selection,
> why not write such a function, and let users who want to code commands
> that use this functionality do that?  IOW, why do we have to come up
> with a command to do something which we aren't sure is a frequent use
> case?

How about having those in subr.el?

 -- Function: overlay-exchange-region overlay
 -- Function: overlay-exists-p overlay
 -- Function: overlay-to-region overlay
 -- Function: overlay-from-region overlay