[Galene] Heads up: Galène generates self-signed certificates

[Juliusz Chroboczek <jch@irif.fr>]

Hi,

While fixing https://github.com/jech/galene/issues/13, I implemented
automatic generation of self-signed certificates if a certificate is not
found in the data/ directory.  In short:

1. If you're currently using a real certificate (stored in data/cert.pem
   and data/key.pem), there's nothing to do.  The only difference is that
   Galène will notice when you update the certificate, and load the new
   certificate automatically.

2. If you're currently using a self-signed certificate, you may remove it:
   Galène will notice that there is no certificate, and generate
   a self-signed certificate in memory (the certificate is never written
   to disk, as usual in Galène).

The self-signed certificate uses 2048-bit RSA, which I understand is
compatible with all browsers.  I could easily generate ed25519 or P-256
instead, if you understand the crypto please let me know what to do.

I've tested that my self-generated certificate works with Chromium and
Firefox.  Please let me know if you think there's something wrong.

-- Juliusz