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/server.c b/src/server.c
index 8d0ae74..47eaee6 100644
--- a/src/server.c
+++ b/src/server.c
@@ -4460,7 +4460,7 @@
 	snprintf(trash.area, trash.size, "%d (initial %d)\n", sv->uweight,
 		 sv->iweight);
 	if (ci_putstr(si_ic(si), trash.area) == -1) {
-		si_applet_cant_put(si);
+		si_cant_put(si);
 		return 0;
 	}
 	return 1;