HTSP: Add support for timeshifting

Currently only pause/play is supported
js: Add showtime.md5digest() and showtime.sha1digest()
fa: Reset filepos when reusing files
opensubtitles: Avoid doing null queries
httpclient: If Range request fails instead of HEAD to query file size switch to streaming mode
Avoid scanning the filesystem for subtitles when playing HLS videos
ps3: Increase stacksize to avoid stack overrun

Fixes #1515
Upgrade libav to v9.1