Integrate link status

[lazyfrosch]

We should add a metric to show the link status.

Some values I've seen during reconnects:

dslmodem_status_info{instance="dsl-modem", job="local", mode="VDSL2", profile="0", status="G.994 Training", traffic_type="Inactive"}
dslmodem_status_info{instance="dsl-modem", job="local", mode="VDSL2", profile="0", status="Idle", traffic_type="Inactive"}
dslmodem_status_info{instance="dsl-modem", job="local", mode="VDSL2 Annex B", profile="0", status="G.993 Started", traffic_type="Inactive"}
dslmodem_status_info{instance="dsl-modem", job="local", mode="VDSL2 Annex B", profile="Profile 17a", status="Showtime", traffic_type="PTM Mode"}