Add payload, probe, and matchline for svrloc

Service Location Protocol version 2 specified by RFC 2608. Partially
implemented by srvloc.lua library. Probe checks for
service:service-agent, which should be implemented by all Service Agents
(servers). Match line only matches version 2, since I don't have any
other versions to test. Expect we will get more service fingerprints to
clarify.