modemmanager: report network initiated disconnections to netifd

modemmanager: report network initiated disconnections to netifd

The new connection dispatcher scripts support integrated in
ModemManager 1.18.8 allows us to provide a openwrt-specific dispatcher
script used to report netifd that the underlying network connection is
down.

See also https://gitlab.freedesktop.org/mobile-broadband/ModemManager/-/merge_requests/775

Fixes https://github.com/openwrt/openwrt/issues/8368
Fixes https://github.com/openwrt/packages/issues/14096

Signed-off-by: Aleksander Morgado <aleksander@aleksander.es>