On Linux 2.6.8 and older, epoll_ctl(EPOLL_CTL_DEL) requires a poll_event

On Linux 2.6.8 and older, epoll_ctl(EPOLL_CTL_DEL) requires a poll_event
even though it is not used, and will return EFAULT if one is not provided.

Also, instead of silently accepting EBADF from epoll_ctl(), check that
sp->fd != -1 before calling vca_del().  This can happen in some cases
where vca_pollsession() closes the session before returning.  This way,
we will still get an assertion failure if epoll_ctl() fails for some
other (unexpected) reason.

Thanks to airmax for his assistance in tracking this down.

git-svn-id: svn+ssh://projects.linpro.no/svn/varnish/trunk@1143 d4fa192b-c00b-0410-8231-f00ffab90ce4