command: let playlist-play-index current preserve file-local-options

If you set file-local-options/foo at runtime and run playlist-play-index
current or playlist-play-index $current_pos, file-local-options/foo is
reset. Add a flag to make it preserve file-local-options.

The advantage of this change is that it allows binding Ctrl+r set
file-local-options/start ${=time-pos}; playlist-play-index current yes
to reload the current file while preserving all options, independently
of --no-config, --watch-later-options or --reset-on-next-file, and
without writing watch later files to disk. It also preserves
file-local-options set on the CLI unlike loadfile.