loadfile: avoid infinite playlist loading loops

There are a number of ways one can craft a playlist file that refers to
itself or cleverly goes around in a loop to other playlist files. There
is obviously no use for this, but mpv spins around forever trying to
load the files so you have to just SIGTERM/SIGKILL it. We can be smarter
about this and attempt to detect it. The condition for detecting this is
surprisingly simple: the filename of the first entry in the playlist
must match a previous playlist path we stored. If we get this, we can
then log an error and stop playback. If there is a "real" file loaded at
any point in time, then we know it's not an infinite loop and clear out
the saved playlist paths. Fixes #3967.

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