callbacks: Add ssh_threads_get_default() to the callbacks.h

Fixes: T154

Signed-off-by: David Wedderwille <davidwe@posteo.de>
Reviewed-by: Anderson Toshiyuki Sasaki <ansasaki@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

include/libssh/callbacks.h

@@ -944,9 +944,20 @@ LIBSSH_API int ssh_threads_set_callbacks(struct ssh_threads_callbacks_struct
     *cb);
 
 /**
- * @brief returns a pointer on the pthread threads callbacks, to be used with
+ * @brief Returns a pointer to the appropriate callbacks structure for the
+ * environment, to be used with ssh_threads_set_callbacks.
+ *
+ * @returns A pointer to a ssh_threads_callbacks_struct to be used with
  * ssh_threads_set_callbacks.
- * @warning you have to link with the library ssh_threads.
+ *
+ * @see ssh_threads_set_callbacks
+ */
+LIBSSH_API struct ssh_threads_callbacks_struct *ssh_threads_get_default(void);
+
+/**
+ * @brief Returns a pointer on the pthread threads callbacks, to be used with
+ * ssh_threads_set_callbacks.
+ *
  * @see ssh_threads_set_callbacks
  */
 LIBSSH_API struct ssh_threads_callbacks_struct *ssh_threads_get_pthread(void);