xmlrpcpp should use poll() for solving 1024 (FD_SETSIZE) clients problem in Linux

xmlrpcpp uses select() for checking the open file descriptors, but this function can't check morethan FD_SETSIZE  file descriptors in Linux. FD_SETSIZE is set to 1024 by default and increasing that value would require to rebuild the current kernel.

select's 'successor' poll() isn't limited by 1024, so replacing select() by poll() would fix that problem.
