isisd: API: area (L1), domain (L2) passwords

Last isisd CLI cleanup for now.  This also folds L1 & L2 configs into
common functions, reducing CLI function bloat by a bit.

(This patch contains changes authored by both Christian Franke and David
Lamparter.)

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
3 files changed