rework pa_once once again, because the once function needs to have terminated before...

rework pa_once once again, because the once function needs to have terminated before pa_once returns, regardless whether the local call executes it or another thread does.

With the previous code it might happen that an long-running initializing in a once function is not terminated yet when another thread thinks it already is.

git-svn-id: file:///home/lennart/svn/public/pulseaudio/trunk@1387 fefdeb5f-60dc-0310-8127-8f9354f1896f