Fix XMPP nickname parsing of special characters

[poVoq]

I was asked by someone if we could backport the changes proposed in this upstream PR:

42wim#2229

@maidl0ver could you please open a new PR here with those changes?

[selfhoster1312]

Ok i looked it up in order to have a correct implementation we need to implement RFC6885 stringprep/PRECIS with the "resource" profile.

That PR you linked to is therefore still not correct. We could still merge it with a comment explaining why it's not standards-compliant, which would already be an improvement.

[jugendhacker]

I think we could actually get aways without implementing stringprep completely, by doing something similar to mellium