Deterministic offline username and UUID

[mindstorm38]

Currently, a new UUID (v4, random one) is used for offline start if -i option is not used and the username is just a sub string of the UUID. This behavior is often annoying (multiple starts, and you can lose your stuff, on offline-mode servers at least) and rarely useful (if you want multiple bots).

The fix will include two main cases:

• No username is given (using -u): a UUID from a local computer constant like host name or something like this, the username will be a sub string of the UUID.
• A username is given: the UUID will be generated from the username, using a hash function.

[mindstorm38]

Implemented in 2.0.6.