MINOR: stream-int: rename si_applet_{want|stop|cant}_{get|put}

It doesn't make sense to limit this code to applets, as any stream
interface can use it. Let's rename it by simply dropping the "applet_"
part of the name. No other change was made except updating the comments.
diff --git a/src/memory.c b/src/memory.c
index c4ba54e..ced9af5 100644
--- a/src/memory.c
+++ b/src/memory.c
@@ -514,7 +514,7 @@
 
 	dump_pools_to_trash();
 	if (ci_putchk(si_ic(si), &trash) == -1) {
-		si_applet_cant_put(si);
+		si_cant_put(si);
 		return 0;
 	}
 	return 1;