A.support soft link better
before this patch
  "kubectl cp" command will copy the soft link to destination as an empty regular file
after this patch
  "kubectl cp" command will behave the same as tar command
  this patch improves it on both from container and to container

B.fix some bugs
1.from container to host
  a.when copy a file ends with '/', it will cause a panic.
    for example, container gakki has a regular file /tmp/test, then run command
    kubectl cp gakki:/tmp/test/ /tmp
    panic happens
  b.when copy a file which does not exist in container, the command ends up without
    any error information

2.from host to container
  a.when run command like
    kubectl cp "" gakki:/tmp
    it will try cp current directory to container, in other words, this command works
    the same as kubectl cp . gakki:/tmp
  b.current cp command will omit an empty directory

modified:   pkg/kubectl/cmd/cp.go
modified:   pkg/kubectl/cmd/cp_test.go