Notify dumped network interfaces

[dulive]

Hello,

Currently mptcpd does not notify nor allows to notify plugins about network interfaces received in the initial dump, so if a plugin needs to initialize something or take some action before receiving/handling its addresses it has to do it when it receives the dumped addresses with some kind of verification, that sometimes can be annoying to get right, and after the initial dump end ups being useless.

So, with this, it would be useful if plugins could receive notifications about the network interfaces obtained in the initial dump, possibly using a new notify-flag.