stream: re-add accidentally removed seek call

This was under CONFIG_NETWORKING, so in theory it should have been save
to remove. But actually this disables forward skipping when reading
from a pipe. (Still a questionable feature, because it doesn't behave
well with libavformat - but it was not supposed to be changed.)
stream: remove weird STREAMTYPE_STREAM special handling

This was an old leftover from an earlier cleanup (which happened in
2003), and which used "special" stuff for streams that could be only
forward-seeked.

Also, don't add mode flags to s->flags; they're supposed to be in
s->mode instead.
Remove some leftovers from network removal

stream_vstream.c in particular was actually dependent on the network
code, and didn't compile anymore.

Cleanup the protocol list in mpv.rst, and add some missing ones
supported by libavformat to stream_lavf.c.
stream: don't treat position 0 specially

Seeking to position 0 meant to try reconnecting with some streams,
actually just the internal http implementation. This has been removed,
so we don't need the special handling anymore.

This means we don't have to be stuck in a retry loop if the stream
doesn't even support reconnect.