Always mark pipe in pipe-open as inherit-on-exec

Always mark pipe in pipe-open as inherit-on-exec

Since 2cdf406a a lot of file descriptors are opened close-on-exec,
including the pipe that is passed to the child process in a pipe-open.
This is usually fine because a dup2 follows to rename that handle to
stdin/stdout that will set the inherit-on-exec. However, if the pipe
descriptor already has the right value, for example because stdin was
closed, then no dup2 happens and hence it's still marked as
close-on-exec right when we want to perform an exec.

This patch explicitly marks such a handle as inherit-on-exec, to ensure
it will be open for the child process.