timer: remove unneeded time_t overflow check

This is mostly covered by the OSX workaround, if the timeout is very
high. It also means that with systems using 32 bit time_t, the time will
overflow 2036 already, instead of 2037, but we don't consider this a
problem.
player: don't init/uninit terminal at runtime

This seems like an unnecessary complication.
stream: kill start_pos, remove --sb option

stream.start_pos was needed for optical media only, and (apparently) not
for very good reasons. Just get rid of it.

For stream_dvd, we don't need to do anything. Byte seeking was already
removed from it earlier.

For stream_cdda and stream_vcd, emulate the start_pos by offsetting the
stream pos as seen by the rest of mpv.

The bits in discnav.c and loadfile.c were for dealing with the code
seeking back to the start in demux.c. Handle this differently by
assuming the demuxer is always initialized with the stream at start
position, and instead seek back if initializing the demuxer fails.

Remove the --sb option, which worked by modifying stream.start_pos. If
someone really wants this option, it could be added back by creating a
"slice" stream (actually ffmpeg already has such a thing).
stream: don't use end_pos

Stop using it in most places, and prefer STREAM_CTRL_GET_SIZE. The
advantage is that always the correct size will be used. There can be no
doubt anymore whether the end_pos value is outdated (as it happens often
with files that are being downloaded).

Some streams still use end_pos. They don't change size, and it's easier
to emulate STREAM_CTRL_GET_SIZE using end_pos, instead of adding a
STREAM_CTRL_GET_SIZE implementation to these streams.

Make sure int64_t is always used for STREAM_CTRL_GET_SIZE (it was
uint64_t before).

Remove the seek flags mess, and replace them with a seekable flag. Every
stream must set it consistently now, and an assertion in stream.c checks
this. Don't distinguish between streams that can only be forward or
backwards seeked, since we have no such stream types.
stream_lavf: remove redundant message prefixes
stream: remove chaos related to writeable streams

For some reason, we support writeable streams. (Only encoding uses that,
and the use of it looks messy enough that I want to replace it with FILE
or avio today.)

It's a chaos: most streams do not actually check the mode parameter like
they should. Simplify it, and let streams signal availability of write
mode by setting a flag in the stream info struct.
stream_file: readjust some windows ifdeffery

Also sneak in some cosmetics.

setmode() exists on Windows/msvcrt only, so there's no need for a
config test.

I couldn't reproduce the problem with seekable pipes on wine, so axe
it. (I'm aware that it still could be an issue on real Windows.)
video: add --video-rotate option for controlling auto-rotation
client API: fix mpv_observe_property with MP_FORMAT_NONE

It returned only 1 change event (after registration), and then went
silent. This was accidentally broken some time ago.