Support Mosquitto's sha512-pbkdf2 password file format

[halfgaar]

TODO:

• Implement.
• Add warning in man page that with thousands or millions of clients, purposefully slow hashing algorithms can be a problem, because it may require hours for all clients to connect.

Example:

wiebe:$7$101$oW4vum9VREb/0aH6K3ge7kY9u26JboPDpF+9SKjd0fdz6SXidJ9n+TtBbg5n0ImfM/LkIEn6BZxT78Wn6Uzgvw==$Sc31amckhadG9Sd7/qTR8L+ROxljBLCMTrFj/Isb0rikvolsWnucdfDDY3bcIk03Nm/ymzfenE7LuTtgbz3wkA==

[halfgaar]

Add warning in man page that with thousands or millions of clients, purposefully slow hashing algorithms can be a problem, because it may require hours for all clients to connect.

That point is actually not (that) relevant in this case. Contrary to bcrypt, which can take 100 ms, a quick test showed 10000 calls with Mosquitto's 101 iterations took 0.7 s.

Fix merged in master, pending new version release.

[halfgaar]

Fixed with release 0.13.1.