libavformat: Rename the applehttp protocol to hls

Keep the old protocol name around for backwards compatibility
until the next bump.

Deprecate the method of implicitly assuming the nested protocol.
For applehttp://server/path, it might have felt logical, but
supporting hls://server/path isn't quite as intuitive. Therefore
only support hls+http://server/path

 from now on.

Using this protocol at all is discouraged, since the hls demuxer
is more complete and fits into the architecture better. There
have been cases where the protocol implementation worked better
than the demuxer, but this should no longer be the case.

Signed-off-by: Martin Storsjö <martin@martin.st>