pjlib-test: edit ssl_sock to use modern ciphers

DES is very old and vulnerable and should not be used as test element.
Choose the closest cipher available (according to the protocol supported)
in order to support more backends.
ssl_sock: add gnutls backend

This backend is mutually exclusive with the OpenSSL one, but completely
compatible, and conformant to the PJSIP API. Also avoids any license issues
when linking statically.

The configure script is updated to select either OpenSSL or GnuTLS
with --enable-ssl[='...'] and a new symbol (PJ_HAS_TLS_SOCK) is introduced
to identify which backend is in use.