public/io/netif.h: change semantics of "request-multicast-control" flag

public/io/netif.h: change semantics of "request-multicast-control" flag

My patch b2700877 "move and amend multicast control documentation"
clarified use of the multicast control protocol between frontend and
backend. However, it transpires that the restrictions that documentation
placed on the "request-multicast-control" flag make it hard for a
frontend to enable 'all multicast' promiscuous mode, in that to do so
would require the frontend and backend to disconnect and re-connect.

This patch adds a new "feature-dynamic-multicast-control" flag to allow
a backend to advertise that it will watch "request-multicast-control" hence
allowing it to be meaningfully modified by the frontend at any time rather
than only when the frontend and backend are disconnected.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Jan Beulich <jbeulich@suse.com>
Cc: Keir Fraser <keir@xen.org>
Cc: Tim Deegan <tim@xen.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>