Traffic Manager when autoconf fd > 32 will cause endless spins causing traffic_cop to kill server -- Backport 3.2.x

WebIntrMain.cc uses a hard coded FD_SET Size of 32, so when the autoconf file descriptor is greater than 32 it will cause select to sleep forever since no timeout is specified. We had a few systems that would have continuous traffic_server restarts because they would (have a fd > 32) and would never accept the connection request for /synthetic.txt causing cop to kill mananger and server. But when Manager restarted is still got a high fd and it would happen again, while the whole time traffic_server is running just fine. This should be switched to FD_SETSIZE if available.

Discovered by Thomas Jackson