This is semi-standard and doesn't require the util/Compiler.h header.

dup2 returns new_fd on success, not 0

Found with modernize-use-auto
Signed-off-by: Rosen Penev <rosenp@gmail.com>

My concept with `class CancellableOperation` doesn't work properly,
because the kernel may continue to write to the given buffer as soon
as the read finishes.

To fix this, this commit adds `class ReadOperation` which owns the
buffer and the `struct iovec`.  Instances of this class persist until
the read really finishes, even if the operation is canceled.