Emit shell prompt even when in non-interactive mode.

Change-Id: If85c509efe13e4bdb40216c79d560ea7f83ba813
diff --git a/sh/input.c b/sh/input.c
index bfb80f4..056ee8b 100644
--- a/sh/input.c
+++ b/sh/input.c
@@ -175,6 +175,9 @@
 	return pgetc_macro();
 }
 
+int in_interactive_mode() {
+    return parsefile != NULL && parsefile->fd == 0;
+}
 
 static int
 preadfd(void)