syscall: use pipe2 on GNU/Linux for O_CLOEXEC #2656
Description
On GNU/Linux os.Pipe should use the pipe2 system call when available, passing in O_CLOEXEC. pipe2 is new as of Linux kernel version 2.6.27. This can avoid a call to syscall.ForkLock.RLock. Also should use pipe2 in syscall.ForkExec.