Update user_cygwin to 20130313.

* let verbose output to stderr, so that such output is mixed to the command
  output (for example, using redirect).
* avoid to use `flock', it is not a part of base.
* useradd
  * add skel support
  * set homedir properly
  * avoid to try dropping from "Users" when uid=gid (pseudo group as user).
  * also chgrp gid for created homedir.
* userdel
  * catch up error correctly when trying to delete Windows users.

(obache)