vicfg: Deleting configurations now resets to the default, where before it has no effect.

Added a Default instance for TrustLevel, and was able to use that to clear
up several other parts of the code too.

This commit was sponsored by Stephan Schulz

doc/todo/vicfg_comment_gotcha.mdwn

@@ -9,8 +9,12 @@ but that way lies madness. Also, it's not at all clear what the "default"
 should be in response to such an action. The default varies per type of
 configuration, and vicfg does't know about defaults.
 
+> [[fixed|done]]; this was a job for Data.Default!  --[[Joey]]
+
 Instead, I think it should detect when a setting provided in the input
 version of the file is not present in the output version, and plop the user
 back into the editor with an error, telling them that cannot be handled,
 and suggesting they instead change the value to the value they now want it
 to have.
+
+> Nah, too complicated.