Add an optional flag to export circuit id per port

[ppopth]

Previously we have HiddenServiceExportCircuitID which allows us to export the circuit id using haproxy. However, such directive is applied globally in the service. We need a way to export the circuit id only for some virtual ports.

We can do that by adding an optional flag to the HiddenServicePort directive to specify that we will export the circuit id for such port. The flag in HiddenServicePort will override the one in HiddenServiceExportCircuitID, if it's not none. But it will use the protocol specified in HiddenServiceExportCircuitID, if it's otherwise.

So, the new syntax for HiddenServicePort directive will be HiddenServicePort VIRTPORT [TARGET [EXPORT-CIRCUIT-ID-PROTOCOL]]