Expose the main event loop so modules can use it.

Initialize, but do not start, event loop earlier.

Do it through a new API function that leaves room for enhancements
such as a second thread/event loop for modules that can do their
events in a threadsafe manner.  But don't actually do that yet.
Add rlm_unbound a wrapper around libunbound
Add libunbound to travis
Minor reformatting and tweaks