net: conn_mgr: Support Auto-Down

To reduce the amount of boiler-plate needed in applications, this commit
grants conn_mgr the ability to automatically take ifaces that have given
up on connecting into the admin-down state.

Tests adjusted as appropriate.

This behavior can be disabled globally by disabling
NET_CONNECTION_MANAGER_AUTO_IF_DOWN, or disabled per-iface using the
CONN_MGR_IF_NO_AUTO_DOWN flag.

Signed-off-by: Georges Oates_Larsen <georges.larsen@nordicsemi.no>