Switch off down-programmer to avoid overheating

[prasimix]

We need to monitor output power and if its go below DP_NEG_LEV that is negative value in Watts (default -1 W), and that condition lasts more then DP_NEG_DELAY seconds (default 5 s), down-programmer circuit has to be switched off.
Also an error 500,"Down-programmer switched off" has to be generated and in Event view a new warning message has to be inserted: DProg is turned off
Turning channel output off and on will return down-programmer circuit into previous state.

[prasimix]

Tested with build b3ef056, it works fine just need to change message type and text for the event view: it should be warning instead of error and text is DProg CHx is turned off or DProg CHx disabled