Import the first of the new libevent manual pages

Import the first of the new libevent manual pages
written by Ted Bullock <tbullock at comlore dot com>
in order to polish it in the tree.
Not yet linked to the build.

In particular, this documents the so far undocumented functions
event_base_new(3) and event_reinit(3) and provides lots of new
information regarding event_init(3) and event_base_free(3).

Also using input from nicm@, jmc@, and myself, OK nicm@ jmc@.