[MINOR] add support for the "backlog" parameter

[MINOR] add support for the "backlog" parameter

Add the "backlog" parameter to frontends, to give hints to
the system about the approximate listen backlog desired size.

In order to protect against SYN flood attacks, one solution is
to increase the system's SYN backlog size. Depending on the
system, sometimes it is just tunable via a system parameter,
sometimes it is not adjustable at all, and sometimes the system
relies on hints given by the application at the time of the
listen() syscall. By default, HAProxy passes the frontend's
maxconn value to the listen() syscall. On systems which can
make use of this value, it can sometimes be useful to be able
to specify a different value, hence this backlog parameter.