ibuf is just too confusing since there is also the 'struct ibuf'.

This makes using amused a bit nicer on termux where /tmp/ can't be written (or doesn't even exists.) amused still uses /tmp/ if TMPDIR is not specified.

Add a proper upper limit (4096 seems enough for now) and safely NUL-terminate the rbuf. The idea is to later drop the clt->buf static buffer and allocate it on demand only when we need to do (http) chunking.

Generate the HTML for the filter input in js (it's useless to have it there otherwise) and actually filter the entries. It's a basic case insensitive match, but it's a start. To do so, add dispatch event type X: to mark the end of the playlist so js can replay the filtering.

This allows amused-web to stay in sync using websockets (much like existing amused clients watch `amused monitor') and sends the forms in the background to avoid refreshing the page. Still missing is reopening the websocket, maybe blocking the UI in that case and showing the progress.

Keep a copy of the playing queue, status etc. to render the UI without doing imsg in the render path.

In the future we could apply some caching to these.