player: reduce default demuxer cache

A (hopefully) temporary hack to make stream switching delays tolerable.
It's not clear how this should be handled (either executing a precise
seek on track switching, or always enabling all streams), so get this
issue out of the way for now by picking a rather low value.
cache_file: add a mode that creates a temporary file

Since we have to be portable, our options for creating temporary files
are somewhat limited. tmpfile() happens to be available everywhere, so
use that. This function doesn't allow having a "visible" filename or
location, so we use the magic string "TMP" for this.