bfdd: make sessions administratively up by default

bfdd: make sessions administratively up by default

Current behavior is inconsistent. When the session is created by another
daemon, it is up by default. When we later configure peer in bfdd, the
session is still up, but the NB layer thinks that it is down.

More than that, even when the session is created in bfdd using peer
command, it is created in DOWN state, not ADM_DOWN. And it actually
starts sending and receiving packets. The sessions is marked with
SHUTDOWN flag only when we try to reconfigure some parameter. This
behavior is also very unexpected.

Fixes #7780.

Signed-off-by: Igor Ryzhov <iryzhov@nfware.com>