Store password hashes prefixed with the used scheme/algorithm

There are several open issues and pull requests around password hashing. However we decide what (currently) a decent default setting is, being able to change hashing functions by configuration is a good thing.

Dovecot allows to store the password algorithm as prefix of the hashed password (additionally it can be suffixed with the salt), which is nice way to be able to deal with different hashing functions in use at the same time. That is mandatory if we want to auto-upgrade hashes the next time a users logs in with the clear-text password like mentioned by @LefsFlarey.

To ensure compatibility with existing installs the configured default algorithm is used when there the stored password is not prefixed at all.

After some feedback, I'd love to start working on a pull-request for this feature. So please let me know what you think and which hashes should be supported. Maybe there's even a library that already does that?

Start by reviewing the linked issue #1734, pull request #3642, and Dovecot password-scheme reference to understand the existing hashing configuration and compatibility requirements. Done means hashes identify their scheme or algorithm while unprefixed existing hashes continue using the configured default, with the supported schemes agreed and covered by tests.