feat: Add config-ssh command (#735)

* feat: Add config-ssh command

Closes #254 and #499.

* Fix Windows support

agent/usershell/usershell_other.go

@@ -27,5 +27,5 @@ func Get(username string) (string, error) {
 		}
 		return parts[6], nil
 	}
-	return "", xerrors.New("user not found in /etc/passwd and $SHELL not set")
+	return "", xerrors.Errorf("user %q not found in /etc/passwd", username)
 }