Replace GnuTLS by OpenSSL

[igsha]

Motivation

Native building through msvc on windows is possible with openssl library.
It is recommended to use meson because it is native tool to build vala and glib-oriented projects.

What works

1. Dino can be built on windows using mingw + cmake or meson + gnutls.
2. Dino can be built though meson with openssl but w/o ICE plugin. That's why calling does not work now.

Analysis

The GnuTLS library is used in the ICE plugin to establish encrypted session.
There are 2 parts in the plugin (dtls_srtp.vala):

• Certificate generation and maniplation through X509.
• Session manipulation:
  • create TLS connection via UDP (DTLS);
  • extract certificates from session;
  • handshaking;
  • support SRTP profile;
  • set priority profile string, e.g., NORMAL:!VERS-TLS-ALL:+VERS-DTLS-ALL:+CTYPE-CLI-X509.

It seems all of the above features are supported by openssl.
There is no much information about DTLS handshaking between openssl and gnutls.
The first part (X509) can be easily ported on openssl.

The most unlear part is SRTP support, set priority string and handshaking.