Add audio/x-flac MIME type

Change streaming FLAC MIME type from application/octet to audio/x-flac to
support Frontier Silicon systems (such as the Axis X3) that require
this. (Others don't care.)

FLAC not added to DefaultDlnaMap as * suffices.

(Commit amend: tabs --> spaces)
(Commit amend: flac --> x-flac)