Now that we raised the minimum GnuTLS dependency to 3.4.6, we can drop workarounds:
- 3.1.9 (2013-02-27) would let us replace gnutls_transport_get_ptr() with gnutls_transport_get_int() (likewise for set functions), avoiding the need for casts. This holds as long as the sockets use TCP (SOCK_STREAM); currently they all do.
- 3.3.0 (2014-04-10) would let us drop the call to gnutls_global_init().
- 3.4.6 (2015-10-20) would let us remove verify_peer_cert() and use gnutls_session_set_verify_cert() instead.