config_transport: Tell pjproject to allow all SSL/TLS protocols

The default tls settings for pjproject only allow TLS 1, TLS 1.1 and TLS 1.2.
SSL is not allowed.   So, even if you specify "sslv3" for a transport method,
it's silently ignored and one of the TLS protocols is used.  This was a new
behavior of pjsip_tls_setting_default() in 2.4 (when tls.proto was added) that
we never caught.

Now we need to set tls.proto = 0 after we call pjsip_tls_setting_default().
This tells pjproject to set the socket protocol to match the method.

ASTERISK-26004 #close

Change-Id: Icfb55c1ebe921298dedb4b1a1d3bdc3ca41dd078