stream: add a file cache

For remarks, pretty much see the manpage additions. Could help with
network streams that require too much seeking (maybe), or might be
extended to help with the use case of watching and downloading a file
at the same time.

In general, it might be a useless feature and could be removed again.

wscript_build.py

@@ -233,6 +233,7 @@ def build(ctx):
         ( "stream/ai_sndio.c",                   "sndio" ),
         ( "stream/audio_in.c",                   "audio-input" ),
         ( "stream/cache.c" ),
+        ( "stream/cache_file.c" ),
         ( "stream/cookies.c" ),
         ( "stream/dvb_tune.c",                   "dvbin" ),
         ( "stream/frequencies.c",                "tv" ),