WRS: add multiple "shortest_path" services

[daniel]

The idea of WRS was to offer multiple services with different shortest path queries. Currently there is only one "shortest_path" service possible and the name is even hard-coded :-(
Bad practice is this: [258] (... but needed a quick solution)

Good would be something like specify in the mapfile a service type or handler:

[...]
<services>
	<service handler="shortest_path" name="[name]" 
			title="[Add a title]" enable="true">

		<description>[Service description]</description>
		[...]
	</service>
	[...]
</services>
[...]

Then "name" attribute would define the part of the URL, and "handler" the service type.