ldpd: change the sorting algorithm of adjacencies

ldpd: change the sorting algorithm of adjacencies

Now the "show mpls ldp discovery" command will display all LDP
adjancencies sorted by address family, neighbor ID and then type (link
or targeted).

Example:
vtysh# show mpls ldp discovery
AF   ID              Type     Source           Holdtime
ipv4 3.3.3.3         Link     rt2-eth1               15
ipv4 3.3.3.3         Link     rt2-eth2               15
ipv4 4.4.4.4         Link     rt2-eth1               15
ipv6 1.1.1.1         Link     rt2-eth0               15
ipv6 3.3.3.3         Link     rt2-eth1               15
ipv6 3.3.3.3         Link     rt2-eth2               15
ipv6 4.4.4.4         Link     rt2-eth1               15

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