rpc: register 'interface modify settings' rpc call.

Signed-off-by: Emmanuel Roullit <emmanuel.roullit@gmail.com>
dabbad: add 'interface settings modify' rpc call.

Signed-off-by: Emmanuel Roullit <emmanuel.roullit@gmail.com>
libdabba: add function to set interface settings.

Signed-off-by: Emmanuel Roullit <emmanuel.roullit@gmail.com>
dabba: support 'interface modify settings' rpc.

Signed-off-by: Emmanuel Roullit <emmanuel.roullit@gmail.com>
dabbad: apply interface settings when needed

Signed-off-by: Emmanuel Roullit <emmanuel.roullit@gmail.com>