command: if playback position is unknown, make percent-pos unavailable

Before that, it just returned -1.

The print case is inconsistent with that, but I'll leave it for now,
because it's consistent with status line / show_progress behavior.
osc: check for availabillity of percent-pos instead of length

... to decide wether the seekbar should be enabled or not.
player: use more than 1/100 resolution for --term-osd-bar

If the terminal width is large enough the position marker jumps over
several characters because currently pos only increases in 1/100th steps.
player: handle the corner cases in --term-osd-bar correctly

With the old code and with maximum pos (width - 2) one character too
many is drawn.