Fix inconsistencies in access to the change password feature

if you want to use the "change password" feature implemented in passwd.vm, you need:

• either being on your own profile or having global admin right, to see the "Change password" button in the user profile
• either being on your own profile or having (local) admin right on this profile, to be able to use passwd.vm

For discussion details, see http://markmail.org/thread/x7pb2x7gnulplopd