Fix DUNDi message routing bug when neighboring peer is unreachable

Consider a scenario where DUNDi peer PBX1 has two peers that are its neighbors,
PBX2 and PBX3, and where PBX2 and PBX3 are also neighbors.  If the connection
is temporarily broken between PBX1 and PBX3, PBX1 should not include PBX3 in
the list of peers it sends to PBX2 in a DPDISCOVER message, as it cannot send
messages to PBX3.  If it does, PBX2 will assume that PBX3 already received the
message and fail to forward the message on to PBX3 itself.  This patch fixes
this by only including peers in a DPDISCOVER message that are reachable by the
sending node.  This includes all peers with an empty address
(00:00:00:00:00:00) and that are have been reached by a qualify message.

This patch also prevents attempting to qualify a dynamic peer with an empty
address until that peer registers.

The patch uploaded by Peter was modified slightly for this commit.

(closes issue ASTERISK-19309)
Reported by: Peter Racz
patches:
  dundi_routing.patch uploaded by Peter Racz (license 6290)

........

Merged revisions 372417 from http://svn.asterisk.org/svn/asterisk/branches/1.8
........

Merged revisions 372418 from http://svn.asterisk.org/svn/asterisk/branches/10
........

Merged revisions 372419 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@372420 65c4cc65-6c06-0410-ace0-fbb531ad65f3