ldpd: accept notifications during the session establishment process

ldpd: accept notifications during the session establishment process

If we don't do this, we'll never trigger the backoff exponential timer
since it's impossible to distinguish between Initialization NAK's and
general errors.

Also:
* Implement some missing bits from RFC 5036;
* remove superfluous log message in session_shutdown()
  (send_notification() logs that we're sending a fatal notification).

Regression introduced by commit 8819fc3.

Fixes the following ANVL LDP regressions: 6.19 and 6.21.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>