*.el: Misc improvements and fixes w.r.t lexical binding

* jack.el: Enable lexical-binding.

* emms-playlist-limit.el (emms-playlist-limit--limit-playlist):
Use a closure.

* emms-player-mpg321-remote.el (emms-player-ignore-stop): Declare,
so as to fix a mis-compilation in `emms-player-mpg321-remote-notify-emms`.

* emms-last-played.el (emms-last-played-format-date): Use lexical binding.

* emms-browser.el (emms-browser-filter-only-dir)
(emms-browser-filter-only-type, emms-browser-filter-only-recent):
Return a closure.

Patch by Stefan Monnier