osmo_io: Make name optional, add _set_name() API

This allows renaming the iofd at any later point in time. This is useful
for instance if the parent object holding the iofd changes its name.

Change-Id: If2772a3ccaa98616e0189862a49ab0243435e343
diff --git a/src/core/osmo_io_internal.h b/src/core/osmo_io_internal.h
index bdd2ac5..cd620e7 100644
--- a/src/core/osmo_io_internal.h
+++ b/src/core/osmo_io_internal.h
@@ -44,7 +44,7 @@
 	bool to_free;
 
 	/*! human-readable name to associte with fd */
-	const char *name;
+	char *name;
 
 	/*! send/recv (msg) callback functions */
 	struct osmo_io_ops io_ops;