ldpd and Zebra: Expand existing debug commands.

ldpd and Zebra:  Expand existing debug commands.

L2VPN PW are very hard to determine why they do not come up.  The following
fixes expand the existing show commands in ldp and zebra to display a
reason why the PW is in the DOWN state and also display the labeled nexthop
route selected to reach the PW peer.  By adding this information it will
provide the user some guidance on how to debug the PW issue.  Also fixed an
assert if labels were changed for a PW that is between directly connected
peers.

Signed-off-by: Lynne Morrison <lynne@voltanet.io>