BUG/MEDIUM: threads/fd: do not forget to take into account epoll_fd/pipes

BUG/MEDIUM: threads/fd: do not forget to take into account epoll_fd/pipes

Each thread uses one epoll_fd or kqueue_fd, and a pipe (thus two FDs).
These ones have to be accounted for in the maxsock calculation, otherwise
we can reach maxsock before maxconn. This is difficult to observe but it
in fact happens when a server connects back to the frontend and has checks
enabled : the check uses its FD and serves to fill the loop. In this case
all FDs planed for the datapath are used for this.

This needs to be backported to 1.9 and 1.8.