Add SASL support to core.

[ghost]

It has been long awaited, and now it's finally done! I think I didn't forget anything; username is public, but password is private. I tested it on localhost.

So, briefly how it works:

• If neither SASL username nor password are set, it just skips authentication.
• If password is set but username isn't, a warning is given and authentication is skipped.
• If both are set, it authenticates. I won't explain how SASL works here, look at the code. :)
• If SASL is not supported, it will probably just silently fail at authenticating.

Fixed issue from pull request #211.

[progval]

Don't surround = by spaces in argument list.

[ghost]

@progval Fixed.