Very short timeout for select() call in net listener eats up CPU time

The select() call at src/mongo/util/net/listen.cpp:218 is made with a timeout of 10ms, which is unnecessarily small, and, on one of my machines, results in a constant CPU consumption of >1% when mongod is sitting idle.

Eliminate the timeout altogether: just pass NULL and let select() block until something happens. The timeout specified here is unnecessary, as a timeout simply results in select() being called again.