ldpd: set default instance to 1

ldpd: set default instance to 1

LDP opens two sockets to zebra, one through ldpd (always using
instance 0) and another through lde (using whatever instance
was set through the -n command line parameter). If no instance
was set, both connections would use the same protocol and instance,
making it impossible to distinguish them through zserv_find_client.
This meant that a response to a lm connect would erroneously go to
the wrong process. Fix this by having a default instance value of 1,
in case the user does not specify a different one.

Signed-off-by: Emanuele Di Pascale <emanuele@voltanet.io>