vo: fix some nonsense

Commit d38bc531 is incorrect: the 50ms queue-ahead value and the flip
queue offset have different functions. The latter is about calling
flip_page in advance, so the change attempted to show video frames 50ms
in advance on all VOs.

The change was for vo_opengl_cb, but that can be handled differently.
m_option: never return NOPTS value as number from properties

MP_NOPTS_VALUE (basically INT64_MIN) is basically an special timestamp
value that means "unset" or "unknown". Its exact value is internal, and
should never be returned or interpreted by any API.

So return "no" instead (what is also what the parser accepts).
player: set a specific window title in idle mode

Instead of "(unavailable)".
player: update chapter marks when switching files

This wasn't done, so the chapter marks of the previous file were
visible. It's harmless, but looks unpolished, so fix it.
command: fix crash with some forms of script_binding

"script_binding name ..." just crashed it. Oops.
lua: don't ignore key press events for script key bindings

Meh.
options: add slightly more sophistcated mechanism for option deprecation
manpage: fix mistake in --video-zoom mention

Fixes #1331.
pipe-win32: possible fix for Windows XP deadlock

This fixes a hang with the VirtualBox OpenGL drivers. It might help
with #1325 as well.
DOCS/contribute.md: remove unintentional headline

The # from the IRC channel made it appear as a headline.