[phobos] parallelism segfaults

[Martin Nowak]

I've had a look the core dumps from the sometimes failing std.parallelism  
test.
The issue is one of having daemon threads running while the GC is  
unmapping memory.
Usually this goes unnoticed because the parallelism threads wait in a work  
queue condition.
Sometimes a daemon thread is awakening from it's GC suspend handler after  
memory was already
freed. This issue is already mentioned in a comment at gc_term.


Thread  obj = Thread.getThis();

...
suspend
...

if( obj && !obj.m_lock ) // <- segfault


I think we should bluntly kill daemon threads after thread_joinAll.

martin