Add genl_socket_close() function

We've had genl_socket_open() but no corresponding genl_socket_close()
so far.  Let's fix that.
diff --git a/TODO-RELEASE b/TODO-RELEASE
new file mode 100644
index 0000000..24da897
--- /dev/null
+++ b/TODO-RELEASE
@@ -0,0 +1 @@
+* new genl_socket_close() function