conncheck: fix try-alternate on tcp turn sockets

conncheck: fix try-alternate on tcp turn sockets

In OC2007R2 compatibility mode, we observed the behaviour of a skype
turn server, when returning code 300 (try-alternate) stun error on its
tls connections. This value is returned apparently when the turn server
is overloaded already.

We noticed that the actual code in priv_handle_turn_alternate_server()
cannot handle a non-udp turn server, because a tcp one would require
to create a new socket.

But, even when creating such a new socket stack (tcp-bsd socket +
pseudossl socket), libnice still fails to establish a new connection to
the alternate server on port 443, in a very systematic way. I'm not sure
whether this problem is specific to this skype server infrastructure
(the skype client fails in a similar way). Anyway, this code path works
as expected with a non-microsoft turn server (tested with coturn).