Bug#40828: my fault
OK, I forgot to install the 2.1 version of libc6-dev. That fixes the
problem where main() is never reached.
The original problem, where pthread_create() fails, is a little different.
The program calls pthread_attr_setschedparam() as root, but then calls
pthread_create() as the original user. In 2.0, it worked, in 2.1
pthread_create() returns EPERM. I don't know if this is the right thing to
do according to POSIX, but it looks reasonable to me, and I think I can
work around it. So I would recommend closing this bug.