sh(1): print a newline when ^D quits sh

I've always found this a little bit confusing:
> sh
$ ^D> sh
$ ^D>

Reviewed by:	0mp, jilles
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D25813

bin/sh/main.c

@@ -228,6 +228,10 @@ cmdloop(int top)
 		}
 	}
 	popstackmark(&smark);
+	if (top && iflag) {
+		out2c('\n');
+		flushout(out2);
+	}
 }