change the _delay API doc so that it matches its implementation behavior.

This further optimize the performance. 

This is now working properly with a conditional variable bug being fixed (8bbfb5cd).

pthread_cond_timedwait() in PosixCond.hxx:timed_wait(PosixMutex...) returns
EINVAL, if ts.tv_nsec >= 1E9. In this case, it returns to early.

Find attached a patch which fixes this. I chose a compare-subtraction method
to keep ts.tv_nsec below 1E9.

Another option would be
ts.tv_sec += ts.tv_nsec / 1000000000;
ts.tv_nsec %= 1000000000;

But I guess this takes more time on some ARM processors, which don't support
hardware division.

do not hog system device.

which ensures mpd do bit perfect playback on OS X

Switch the remaining users to "postponed_exception".

Got lost in commit 6ed77f2a