Add meshlink_set_canonical_address().

Add meshlink_set_canonical_address().

This introduces the concept of a canonical address for a node, which is
its "official" address/port. It is only set manually, and never updated
or removed when other addresses have been discovered for this node.

Also, this changed meshlink_add_address(mesh, address) to be equivalent to
meshlink_set_canonical_address(mesh, meshlink_get_self(mesh), address, NULL).
While not strictly equivalent to the old behaviour, it matches the
documented intent of this function.