Bug 958554 - Fix about window sometimes saying nightly is being updatd by another instance. r=rstrong

Note that if the process is hung, or there is another instance running in a  different user instance,
or another profile is running, the message is perfectly normal.  But this should prevent the bad case
of getting this message.

This handling was first added specifically for Metro and Firefox running the same intallation at the
same time.  Metro isn't being released but there are still other very valid reasons for having this
handling as noted above.