clear regions before ssh prompt

When built with concurrent-output 1.9, ssh password prompts will no longer
interfere with the -J display.

To avoid flicker, only done when ssh actually does need to prompt;
ssh is first run in batch mode and if that succeeds the connection is up
and no need to clear regions.

This commit was supported by the NSF-funded DataLad project.

doc/bugs/minor_display_glitch_with_ssh_password_prompting_and_-J.mdwn

@@ -47,3 +47,5 @@ Some approaches to fix it:
    See <https://github.com/feuerbach/ansi-terminal/issues/7>
 
 --[[Joey]]
+
+> [[fixed|done]] using option #3. --[[Joey]]