bfdd: implement passive mode

bfdd: implement passive mode

The passive mode is briefly described in the RFC 5880 Bidirectional
Forwarding Detection (BFD), Section 6.1. Overview:

> A system may take either an Active role or a Passive role in session
> initialization.  A system taking the Active role MUST send BFD
> Control packets for a particular session, regardless of whether it
> has received any BFD packets for that session.  A system taking the
> Passive role MUST NOT begin sending BFD packets for a particular
> session until it has received a BFD packet for that session, and thus
> has learned the remote system's discriminator value.  At least one
> system MUST take the Active role (possibly both).  The role that a
> system takes is specific to the application of BFD, and is outside
> the scope of this specification.

Signed-off-by: Rafael Zalamena <rzalamena@opensourcerouting.org>