Fix negative dB value issue with Raspberry

[christianTF]

• Use End of Options tag for amixer
• Remove isdigit for volumes as it does not accept negative values

The UI is logically inconsistent as UI shows percent but code uses dB. Therefore, on Raspberry, the UI shows a range of -10000 to 400 % but is dB.

[christianTF]

I've added a is_digit function recognizing also negative numbers.

[JiriSko]

Good job. Thanks!