Refactor and improve libcontainer and driver

Remove logging for now because it is complicating things
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)

system/calls_linux.go

@@ -1,6 +1,7 @@
 package system
 
 import (
+	"os/exec"
 	"syscall"
 )
 
@@ -16,6 +17,14 @@ func Exec(cmd string, args []string, env []string) error {
 	return syscall.Exec(cmd, args, env)
 }
 
+func Execv(cmd string, args []string, env []string) error {
+	name, err := exec.LookPath(cmd)
+	if err != nil {
+		return err
+	}
+	return Exec(name, args, env)
+}
+
 func Fork() (int, error) {
 	syscall.ForkLock.Lock()
 	pid, _, err := syscall.Syscall(syscall.SYS_FORK, 0, 0, 0)