Visudo now checks for the existence of an editor and gives a sensible

Visudo now checks for the existence of an editor and gives a sensible
error if it does not exist.

The path to the editor for visudo is now a colon-separated list of
allowable editors.  If the user has $EDITOR set and it matches
one of the allowed editors that editor will be used.  If not,
the first editor in the list that actually exists is used.