Add an option to prevent putting the shim in a new mount namespace

This is needed for users on kernel older than 3.18 so they can avoid EBUSY
errors when trying to unlink, rename or remove a mountpoint that is present in
a shim namespace.

Signed-off-by: Kenfe-Mickael Laventure <mickael.laventure@gmail.com>

linux/shim/client_unix.go

@@ -7,8 +7,10 @@ import (
 	"syscall"
 )
 
-var atter = syscall.SysProcAttr{
-	Setpgid: true,
+func getSysProcAttr(nonewns bool) *syscall.SysProcAttr {
+	return &syscall.SysProcAttr{
+		Setpgid: true,
+	}
 }
 
 func setCgroup(cgroupPath string, cmd *exec.Cmd) error {