Revise the kqueue_acceptor with a sledgehammer.

Revise the kqueue_acceptor with a sledgehammer.

Use EV_ONESHOT for all session events to make the kernel delete the
event when it fires, to remove any chance of any race with session
state and kqueue event arming.

For sessions with acceptfilter, this does just what we want (and I
kick myself for not realizing this much sooner).

For sessions where the acceptfilter is not enabled or has given up,
this results in an extra kevent arming operation for each read, but
this is still a much lower overhead than synchrnously deleting
the event when before passing the session on.

Delete all the workarounds and band-aids that had accumulated.

All in all, a win-win-win situation.

I have no idea how many tickets this will close.

git-svn-id: svn+ssh://projects.linpro.no/svn/varnish/trunk@2514 d4fa192b-c00b-0410-8231-f00ffab90ce4