It's a bad idea to have a read callback read a fixed amount; Instead,
we should drain it, or have some way to know that more data is
arrived. Not unsafe in this case, but let's not encourage people to
learn bad habits.
This commit also notes some other stuff to fix.
This test opens a server socket, and forks a child which connects to that
server socket many times. It sets a low number for the max open file limit
to catch any file descriptor leaks.
It would not work on Windows since it uses fork() to be able to create both the
server and the clients.
