Rigctl Add PowerOff , Correct Set AF Gain

[wwortel]

PowerOff allows to stop pihpsdr with saving of settings, followed by OS shutdown. This is useful before cutting power e.g. in remote operation. Set AF Gain did not work properly. The value to be produced from 0-255 is a negative dB value and hence a log() function is required.

[wwortel]

PS command now only shuts donw the Linux OS when pihpsdr is found to be running on a platform with XDMA, e.g. Saturn. tested: Anan G2

[dl1ycf]

Thanks for the suggestions/fixes, I have manually implemented them and cherry-picked to other branches. One comment to the "PS0" command:

• the shutdown command will fail if the user is a non-su-doer. Therefore I added an _exit thereafter.
• the "PS0" response will never be sent, because the program has been stopped before
• I strongly recomment NOT to use PS0 except for emergency situations.