Add -H parameter to pdbedit to update user password from nt-hash hexstring.

[github at samba.org]

New comment by abartlet on Samba Github repository

https://github.com/samba-team/samba/pull/17#issuecomment-148222066
Comment:
If it isn't too much trouble, can you squash the two patches into one?

Also, we really should have tests for this.  If you feel inspired, could you find a suitable test to extend to check this code still works (it is OK to do this in a source3 test if writing one to hook into the AD DC is too hard)