Revert broken part of [6f653c5a2ba30008d62ab589ff5686df8a29ef72].

This was obviously untested as it breaks external ffmpeg build and only changes this code path.
xbmc/cores/dvdplayer/DVDDemuxers/DVDDemuxers.a(DVDDemuxFFmpeg.o): In function `DllAvFormat::av_read_frame_flush(AVFormatContext*):
/home/alexis/xbmc/lib/DllAvFormat.h:121: undefined reference to
`ff_read_frame_flush'

Even if it did not break, the change is wrong anyway.
Rename our local copy of av_read_frame_flush to xbmc_read_frame_flush as suggested by elupus on PR2633.

That way it makes it clear the function is from xmbc and not ffmpeg.
correct date formats. fixes #14286
Merge pull request #2633 from aballier/mess

Revert broken part of [6f653c5a2ba30008d62ab589ff5686df8a29ef72].
[darwin] - revert the revert the revert - well fix the build for darwin broken in a7703fb74603192f087db90c60877eeed1097f66
Merge pull request #2649 from ronie/date

correct spanish date formats. fixes #14286
[Settings] Add m_subtitleExtensions.
[Util] Generalize external subtitle retrieval method(s).

Add void CUtil::GetPathsToLookForAssociatedItems()
Add void CUtil::ScanPathsForAssociatedItems(...)
Add int CUtil::ScanArchiveForAssociatedItems()
[Util] Make use of generalized ScanForExternalSubtitles(...) methods.
ScanForExternalSubtitles(...) now uses GetPathsToLookForAssociatedItems() and ScanPathsForAssociatedItems(...).
Remove obsolete method ScanArchiveForSubtitles() and sub_exts[].
[Util] Add method to scan for external audio files.
[DVDPlayer] Open input streams for ext. audio files and store them.
[DVDPlayer] Add open external demuxers and read from them.
Update the SelectionStream.
Adapt all relevant places where m_pDemuxer is used; in particular ReadPacket(...) and add ReadExternalAudioPacket(...)
Adapt DVDFileInfo so that it reads infos also from external audio files (e.g. language and channels).