init: Start threads on nebmodule_init

This patch makes module initialisation a bit more apparent, by starting
threads on module initialisation instead of waiting for a NEB callback
before starting them.

There's a comment in here which claimes that Nagios daemonizes *after*
having loaded the NEB modules. As far as I can tell, this is simply not
true.

This is mainly about readability/clarity, but it might also solve some
raciness somewhere. I haven't looked too far into that.

Change-Id: Iddd1115749de23fb626043bf5082d58d341af336
Signed-off-by: Anton Lofgren <alofgren@op5.com>