isisd: API: basic circuit config

Create isis_vty.c and start moving off CLI functions into that.  These
then call newly-added "nice" API wrappers.

Patch contains significant work authored by Christian Franke.

[v2: removed stuff that crept in from the next patch]

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/isisd/isis_redist.h b/isisd/isis_redist.h
index 2184620..cc9c2e6 100644
--- a/isisd/isis_redist.h
+++ b/isisd/isis_redist.h
@@ -44,6 +44,7 @@
 
 struct isis_area;
 struct prefix;
+struct vty;
 
 struct route_table *get_ext_reach(struct isis_area *area,
                                   int family, int level);