playlist: use path_strip_last_volume, clarify path conventions

The comment regarding Windows paths was fairly confusing as to
its intent. What the code is trying to do is replace the drive
letter with the volume containing the playlist, which appears
reasonable. The middle of the comment was devoted to explaining
why the code below was potentially incorrect which only served
to add to the confusion.

AFAICT the last volume specifier in a path will cause search to
start from the root of that volume, so any incorrect result can
be avoided by using the new function path_strip_last_volume().

Finally, add a comment at the top of the function that explains
what it does.

Change-Id: If4e4938801f2f81eb52f5d32b5461872995e5e83

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