mu4e: use mu4e~update-name de facto

Instead of hard coding the buffer name " *mu4e-update*", use the
buffer name defined in `mu4e~update-name`.  Also, the default name
defined in `mu4e~update-name` were changed to match the previously
hard-coded invisible buffer name.

mu4e/mu4e-utils.el

@@ -958,7 +958,7 @@ RUN-IN-BACKGROUND is non-nil (or called with prefix-argument),
 run in the background; otherwise, pop up a window."
   (let* ((process-connection-type t)
 	        (proc (start-process-shell-command
-		              "mu4e-update" " *mu4e-update*"
+		              "mu4e-update" mu4e~update-name
 		              mu4e-get-mail-command))
 	        (buf (process-buffer proc))
 	        (win (or run-in-background