Regression from b5662ea9

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

- Create a libvlc instance, in order to init the module bank, before testing if
  modules exist.
- Use mkstemp to create a unique temporary file used by plaintext
- Don't test secret and kwallet by default in order to don't pollute
  developer's keystores (run this test with "-a" argv to test every keystores).

The Chromecast protocol is not a client server, all requests don't get an
answer and we can receive unexpected broadcasts.
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

AddPendingEvent is protected in wxWindow so orignial example code don't compile. Access to this method should be made by using wxWindow::GetEventHandler()
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

Close #16338

Since helpers use p_intf.

If set to true, then the user asked to store the password.

If open fails, either the file doesn't exist or permissions are wrong.

TODO: fix libdsm to return the good error code in case or bad permissions.

Deactivated by default since secret is not encrypted.

To use it (for test purpose only):
./vlc --keystore=plaintext --keystore-plaintext-file=<my_file> <url>

For future plaintext module.

as in 2012 spec