modem: hl7800: ensure net_if_down is triggered and not ignored

Trigger network dropped based on socket error notifications.
This debounces the network state and only triggers
the network interface to go down if the network
really drops or causes socket problems.
This will ensure upper networking stack layers can
cleanup broken sockets properly.

Debounce DNS resolver refresh.
Only trigger DNS resolver refresh if the DNS address changes.

Signed-off-by: Ryan Erickson <ryan.erickson@lairdconnect.com>