This should fix OSX/BSD issues with not noticing transfer information
files with kqueue. Now that threads are used, the thread can manage the
transfer slot allocation and deallocation by itself; much cleaner.
A few places catch IO errors after calling runThreadState,
but since the MVar was not restored, it'd later deadlock trying to read
from it.
I'd like to catch all exceptions here, but I could not get the types
to unify.
