core: Clean up move-to-next-file logic

core: Clean up move-to-next-file logic

The mpctx->eof field was used to also store other reasons to stop
playing the current file besides EOF, and the code didn't properly
distinguish those from EOF. Rename the field to "stop_play" and clean
up some of the code accessing it. Properly separating EOF from other
reasons allows handling file switching better.

Before this commit buffered audio was always drained before moving to
the next file (the only exception when it was NOT drained when
stopping a file was when quitting the player completely). This added
an extra delay when manually moving to the next file as you had to
wait for the currently buffered audio to finish playing before the
next file would start. After this commit audio is only drained if the
file reaches EOF normally; otherwise audio is cut and the next file
starts immediately.