Inconsistent _type definition with multi-arc videos

[ChrisK2]

youtube-dl -J --flat-playlist 'http://southpark.cc.com/full-episodes/s18e07-grounded-vindaloop'
returns "_type": "compat_list"

youtube-dl -J --flat-playlist 'http://thecolbertreport.cc.com/full-episodes/lykr9m/november-18--2014---eva-longoria'
returns "_type": "playlist"

... although both are similarly structured multi-arc videos. For mpv it would be very good to know exactly when we have a multi-arc video, since those require different handling from actual playlists and also enables us to use some neat features like building one long virtual video out of the arcs. So ideally, multi-arc videos should always return "_type": "compat_list" (or something entirely different, but not "playlist").

[phihag]

As indicated by the compat_, results of the type compat_list are using an outdated protocol and are intended to be phased out. Instead, we should either set a property or designate a new _type value.