Fix a problem with reaping a thread with detached children

When a thread that has detached children is reaped the detached children
need to be fully detached so they can be reaped correctly. If they are not
fully detached then they may reference a parent that has already been
reaped (memory freed). Found with valgrind.
Merge branch 'master' of git://github.com/steveicarus/iverilog