Support VISUAL environment variable for editing. (#10898)

If the user has set the environment variable VISUAL, it will be used
in preference to EDITOR for all Spack editing activities. If VISUAL
is not set or fails (perhaps due to a lack of graphical editing
capabilities),EDITOR will be used instead. We fall back to one of
several common editors if neither bears fruit.

This feature has been tailored to:

* Provide identical behavior to the previous implementation in the
  case that VISUAL is not set.
* Not require any change to code utilizing the editor feature.
* Follow usual UNIX behavior concerning VISUAL and EDITOR.