Clean up configure magics selecting which comm loop to use, promote epoll to stable

Clean up configure magics selecting which comm loop to use, promote epoll to stable

This is basically a copy from Squid-2, making configure a bit smarter
in selecting which comm loop to use, and automatically enabling epoll
if it seems usable.

kqueue is still not activated automatically even if defected as the
comm_kqueue implementation is still experimental and known to have issues