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

Since 2cdf406 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.

util.c

@@ -2469,8 +2469,10 @@ Perl_my_popen(pTHX_ const char *cmd, const char *mode)
 	    if (p[THAT] != (*mode == 'r'))	/* if dup2() didn't close it */
 		PerlLIO_close(p[THAT]);
 	}
-	else
+	else {
+	    setfd_cloexec_or_inhexec_by_sysfdness(p[THIS]);
 	    PerlLIO_close(p[THAT]);
+	}
 #ifndef OS2
 	if (doexec) {
 #if !defined(HAS_FCNTL) || !defined(F_SETFD)