Revert "pipe: make read() return when write end is closed"

Oops, this was incorrect. The weak_count test handles this, and this change causes it to return EOF when any file descriptor is dropped, if there are several.

The real cause of the issue I had was because it failed to set O_CLOEXEC with fcntl(), which wasn't clear because it didn't make any system call (since that function wasn't implemented in newlib) and git wasn't checking the return value. That is addressed in https://github.com/redox-os/newlib/pull/40.