SSH: use libssh2_session_handshake()

In libssh2 1.2.8, libssh2_session_handshake() replaces
libssh2_session_startup() to fix the previous portability problem with
the socket type that was too small for win64 and thus easily could cause
crashes and more.

lib/ssh.c

@@ -496,6 +496,13 @@ static int sshkeycallback(CURL *easy,
                                              (libssh2_uint64_t)d, 0, 0)
 #endif
 
+/*
+ * libssh2 1.2.8 fixed the problem with 32bit ints used for sockets on win64.
+ */
+#ifdef HAVE_LIBSSH2_SESSION_HANDSHAKE
+#define libssh2_session_startup(x,y) libssh2_session_handshake(x,y)
+#endif
+
 /*
  * ssh_statemach_act() runs the SSH state machine as far as it can without
  * blocking and without reaching the end.  The data the pointer 'block' points