check if the provided password is really the current log-in password
Merge pull request #11993 from owncloud/enc_repeat_password

[encryption] check if the provided password is really the current log-in password
Add OCP\Security\IHasher

Public interface for hashing which also works with legacy ownCloud hashes and supports updating the legacy hash via a passed reference.

Follow-up of https://github.com/owncloud/core/pull/10219#issuecomment-61624662
Requires https://github.com/owncloud/3rdparty/pull/136