Define and use LIBSSH2_INVALID_SOCKET instead of INVALID_SOCKET

INVALID_SOCKET is a special value in Windows representing a
non-valid socket identifier. We were #defining this to -1 on
non-Windows platforms, causing unneccessary namespace pollution.
Let's have our own identifier instead.

Thanks to Matt Lawson for pointing this out.

src/session.c

@@ -675,7 +675,7 @@ session_startup(LIBSSH2_SESSION *session, libssh2_socket_t sock)
     if (session->startup_state == libssh2_NB_state_idle) {
         _libssh2_debug(session, LIBSSH2_TRACE_TRANS,
                        "session_startup for socket %d", sock);
-        if (INVALID_SOCKET == sock) {
+        if (LIBSSH2_INVALID_SOCKET == sock) {
             /* Did we forget something? */
             return _libssh2_error(session, LIBSSH2_ERROR_BAD_SOCKET,
                                   "Bad socket provided");