Optimization: Fewer epoll(2) system calls when closing a socket (#235)

Optimization: Fewer epoll(2) system calls when closing a socket (#235)

Squid was calling epoll(2) twice to clear a socket interest. One call is
more than enough: Technically, close(2) is supposed to clear epoll(2)
registration for us, but I did not risk relying on that.

In other environments, socket interest changes are pooled together
before being submitted to the OS, so Squid was doing a bit of extra
work, but not making (many) extra system calls AFAICT.

Also fixed (previously unused) Comm::ResetSelect() on these platforms:
* epoll(2): The old resetting code did not clear our interest AFAICT.
* kqueue(2): The old resetting code made no sense to me at all.
* poll(2): There was no code at all.
* select(Win32): There was no code at all.

Even though Comm::ResetSelect() implementation is now the same for all
platforms, I did not make that code platform-agnostic because it is
possible to optimize it further in platform-specific ways.