Fix some mpc.el updating quirks

Fix some mpc.el updating quirks

* lisp/mpc.el (mpc-songs-jump-to): Update the status buffer.

* lisp/mpc.el (mpc-stop): M-x mpc-stop clears playlist queue. So
updating *MPC-Songs* buffer is useful.

* lisp/mpc.el (mpc-cmd-delete): I noticed M-x mpc-playlist-delete
always messages “Deleted 1 songs” even if playlist queue has more
than one songs.  This is because mpc-cmd-delete’s sort modifies
songs-poss by side effect.  Using copy-sequence fixes this (bug#44093).
* lisp/mpc.el (mpc-cmd-move): Ditto.