stream: reconnecting doesn't make sense if stream is not seekable

This stops mpv from being stuck in reconnecting at the end of the file
with some unseekable streams.

Test URL: http://playerservices.streamtheworld.com/pls/CBC_R1_VCR_H.pls
stream: more consistent checks for whether stream is seekable

Never check s->seek (except in init), because it'd have to check
s->flags anyway. Also, for fast skippable streams (like pipes), don't
set the bit that indicates support for seek forward.

Make sure s->end_pos is always 0 for unseekable streams. Lots of code
outside of stream.c uses this to check seeking support.
demux: remove movi_start/movi_end fields

Pointless, using stream->start_pos/end_pos instead.

demux_mf was the only place where this was used specially, but we can
rely on timestamps instead for this case.
demux: make determining seek capability generic

Instead of having each demuxer do it (only demux_mkv actually did...),
let generic code determine whether the file is seekable. This requires
adding exceptions to demuxers where the stream is not seekable, but the
demuxer is.

Sort-of try to improve handling of unseekable files in the player. Exit
early if the file is determined to be unseekable, instead of resetting
all decoders and then performing a pointless seek.

Add an exception to allow seeking if the file is not seekable, but the
stream cache is enabled. Print a warning in this case, because seeking
outside the cache (which we can't prevent since the demuxer is not aware
of this problem) still messes everything up.
player: fix quvi 0.9 playlist loading

The code made no sense at all.