This adds client-side support for the unicode/SAMR password change scheme.

This adds client-side support for the unicode/SAMR password change scheme.

As well as avoiding DOS charset issues, this scheme returns useful error
codes, that we can map back via the pam interface.

This patch also cleans up the interfaces used for password buffers, to
avoid duplication of code.

Andrew Bartlett
(This used to be commit 2a2b1f0c872d154fbcce71a250e23dfad085ba1e)