mainconfig: allow mainconfig logic to be used both internally and externally

corosync logging configuration logic is rather complex and in order
to make it simpler to reuse (at least within corosync/ tree)
we need to be able to use both icmap and cmap.

the patch might seem controversial, but it reduces heaps of code around
from qdevices (coming next).

It might be useful to consider moving this to a common shared library
but there aren't enough users yet and a shared lib would force
corosync to link with cmap (that we do not want at all costs)

Signed-off-by: Fabio M. Di Nitto <fdinitto@redhat.com>
Reviewed-by: Steven Dake <sdake@redhat.com>