player: --save-position-on-quit should always work

Now any action that stops playback of a file (even playlist navigation)
will save the position. Normal EOF is of course excluded from this, as
well as commands that just reload the current file.

The option name is now slightly off, although you could argue what the
word "quit" means.

Fixes #1148 (or at least this is how I understood it).

DOCS/man/options.rst

@@ -290,10 +290,9 @@ Program Behavior
 ``--save-position-on-quit``
     Always save the current playback position on quit. When this file is
     played again later, the player will seek to the old playback position on
-    start. This does not happen if playback of a file is stopped in any other
-    way than quitting. For example, going to the next file in the playlist
-    will not save the position, and start playback at beginning the next time
-    the file is played.
+    start. This happens when playback is stopped in any way by the user.
+    For example, going to the next file in the playlist will save the position,
+    and resume playback at thr same position the next time the file is played.
 
     This behavior is disabled by default, but is always available when quitting
     the player with Shift+Q.

player/core.h

@@ -41,6 +41,10 @@ enum stop_play_reason {
     PT_QUIT,            // stop playback, quit player
 };
 
+// stop_play_reason that is an explicit user abort.
+#define PT_IS_QUIT(x) ((x) == PT_NEXT_ENTRY || (x) == PT_CURRENT_ENTRY || \
+                       (x) == PT_STOP || (x) == PT_QUIT)
+
 enum exit_reason {
   EXIT_NONE,
   EXIT_QUIT,

player/loadfile.c

@@ -1196,7 +1196,7 @@ terminate_playback:
     if (mpctx->stop_play != AT_END_OF_FILE)
         clear_audio_output_buffers(mpctx);
 
-    if (opts->position_save_on_quit && mpctx->stop_play == PT_QUIT)
+    if (opts->position_save_on_quit && PT_IS_QUIT(mpctx->stop_play))
         mp_write_watch_later_conf(mpctx);
 
     if (mpctx->step_frames)