* ampc.el: Refine options for sending commands.
(ampc-send-command): Substitute optional argument unique with key argument list
props.
All callers changed to take advantage of the new options.
(ampc-send-next-command): Handle new command options.
(ampc-skip): Don't skip beyond playlist.
(ampc-fill-status-var): New function.
(ampc-handle-status): Store playlist length. Use ampc-fill-status-var.
(ampc-handle-current-song): Use ampc-fill-status-var.
(ampc-delete): Move point to first deletion. Remove marks.
(ampc-quit): Explicitly delete process.