tv: fix compilation without clock_gettime, don't claim to be MPlayer

mp_msg() doesn't exist anymore in this form. Oops.
tv: remove some non-sense

There's really no need to convert this to float and then back. This is
mostly of cosmetic nature, double precision was probably enough to avoid
rounding.
tv: if timestamp is unset, return NOPTS

Well, not sure if this really improves anything, but at least it's less
of a WTF to the playback core than always returning the same timestamp
for every frame.
demux: use position as signed integer

Seeing (uint64_t)-1 as value when position was unset was annoying.