I created a test program which stresses the interplay between libc,
pthreads and vfs_pipe and may detect regressions.
The program starts a thread that spawns a bunch of workers, sends and
receives data to them through a pipe. When all data of a worker is
collected, the worker is finished (join()) and a new worker is started.
Both the number of bytes sent to the worker and the size of its
answer are random.
Issue #3967
