make network services disable-able

This pull adds:

1. Documentation on the (possibly unintended? :-) ) ability to disable the TCP heartbeat, by providing `0` as the interval; this is useful when one does not have concerns about timing out, and does not want to go through filtering out heartbeats.

2. A similar method to disable some/all of the network services by providing `0` as the port number. This remains undocumented for now (I am not sure where to best document it). (Port `0` <a href="http://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xhtml">is a valid port</a>, but is used here , since (1) it would probably not be selected by users, and (2) numbers above 65535 might be handled incorrectly by code which assumes a 2-byte C integer)

Hopefully the code is clean and bug/exploit free; I am new to this kind of code, arrays of structs and all, so feel free to clean if needed.