ao_alsa: return negative value on error in play()

No functional change, because the only user of ao_play() ignores return
values below 1.
mp_core: sort function prototypes by source file

I considered making a header file for each .c file, but decided against
it. Asking around, not making separate headers was deemed acceptable. In
the end, all of these depend on MPContext and store state inside of it,
so separate headers aren't all that useful anyway.
player: merge mp_osd.h into mp_core.h

Just doing this because mp_osd.h and osd.c is not consistent.

There are some other header files (command.h and screenshot.h), but
since I don't feel too good about inflating mp_core.h, I'm not merging
them, at least not yet.
main: improve a terminal message

Better prefer real English...
x11: minor cosmetics
x11: remove ancient metacity hack

Tested with recent metacity; this code is not triggered anymore. The
code was added in 2003 and probably has been unused for years.
x11: restore support for --wid=0

This stopped working when the code was changed to create a window even
if --wid is used.

It appears we can't create our own window in this case, because in X11
there is no difference between a window with the root window as parent,
and a window that is managed by the WM. So make this (kind of worthless)
special case use the root window itself.
ta: track location debug info in 2 more cases

Was overlooked.