Re-work the adding/removing of ldp_if and ldp_addr.

Re-work the adding/removing of ldp_if and ldp_addr.
Re-work the relationship between ldp_outlabel and ldp_nexthop.

ldp_if can be created due to a new ldp_entity being created or
from a new ldp_fec being added to the system.

ldp_addr can be created due to a new ldp_fec being added to the
system or by processing a ADDR message.

Now ldp_addr and ldp_if are handled differnt then the rest of the
structures.  Creating them automatically put them in the global
list.  Releasing the last refcnt will remove it from the global
list and delete it.

[git-p4: depot-paths = "//depot/ldp-portable/": change = 387]