Convert to libevent.

This is a quick initial conversion that doesn't yet show much advantage:
- We roll our own timeouts.
- We roll our own signal handling.
- We build up the meta connection fd events on each loop rather than
  on state changes.

configure.in

@@ -86,6 +86,9 @@ fi
 
 dnl Checks for libraries.
 
+AC_CHECK_LIB(event, event_init,
+  [], [AC_MSG_ERROR(libevent is required)])
+
 dnl Checks for header files.
 dnl We do this in multiple stages, because unlike Linux all the other operating systems really suck and don't include their own dependencies.