mpdstats: Don't record a skip when stopping MPD.

[aereaux]

Description

MPD keeps the current track in the queue when stopping, so it's not
really like a skip, and I use it so that I can stop the music, and later
start at the beginning of a track.

I do this by keeping track of the current song id, and then comparing
them when we receive a stop signal.

Fixes #3722 .

To Do

• Documentation. (If you've add a new command-line flag, for example, find the appropriate page under docs/ to describe it.)
• Changelog. (Add an entry to docs/changelog.rst near the top of the document.)
• Tests. (Encouraged but not strictly required.)

[sampsyo]

Seems pretty good to me! Would you mind adding a changelog entry describing the change?

[aereaux]

Added.

[sampsyo]

Excellent; thank you!