stream_lavf: print lavf options that could not be set

Mainly for debugging. Usually, we just set options for all possible
protocols, and we can't really know whether a certain protocol is used
beforehand. That's also the reason why avio_open2() takes a dictionary,
instead of letting the user set options directly with av_opt_set(). Or
in other words, we don't know whether an option that could be set is an
error or not, thus we print the messages only at verbose level.
network: fix rtsp playback

By default, libavformat uses UDP for rtsp playback. This doesn't work
very well. Apparently the reason is that the buffer sizes libavformat
chooses for UDP are way too small, and switching to TCP gets rid of this
issue entirely (thanks go to Reimar Döffinger for figuring this out).

In theory, you can set buffer sizes as libavformat options, but that
doesn't seem to help.

Add an option to select the rtsp transport, and make TCP the default.

Also remove an outdated comment from stream.c.
mplayer: attempt to make playback resume work with DVD/BD

The problem with DVD/BD and playback resume is that most often, the
filename is just "dvd://", while the actual path to the DVD disk image
is given with --dvd-device. But playback resume works on the filename
only.

Add a pretty bad hack that includes the path to the disk image if the
filename starts with dvd://, and the same for BD respectively. (It's a
bad hack, but I want to go to bed, so here we go. I might revert or
improve it later, depending on user feedback.)

We have to cleanup the global variable mess around the dvd_device.
Ideally, this should go into MPOpts, but it isn't yet. Make the code
paths in mplayer.c take MPOpts anyway.