Permissions are incorrect on home directory move

PR: https://github.com/freenas/freenas/pull/5362

In case of moving location of home directory from path a to path b, if path b
does not exist, we need to explicitly set permissions as well strip any ACL
on the path in order to ensure that premissions from original path are preserved.

Preference for new mode is as follows:
1. user-supplied mode in 'user.update' call
2. existing mode on old path
3. default mode. Since this is most likely an error condition, default is 0700.