Add Filter for user_name length in ms user signup

[hughwillfayle]

It is difficult for us to explain our clients why they cannot have usernames with 3 chars and left. For that issue I'd like to recommend a filter for that, so we can change this with a plugin.

Diff for #20107

The wpmu_validate_user_signup() function has some restrictions that aren't present when a user registers on a single-site install, so maybe we should just get rid of them.

Should I work on some more diffs?

No, we should hear from a core dev first.

I hope that there will be a response before the freeze. Thanks for you feedback :)

We should probably figure out a way for all of the issues in wpmu_validate_user_signup() to be filtered en masse. This is not the first problem here.

The reason for this restriction is in subdomain situations, keeping it above three letters allows a namespace for datacenters, country/locale codes, etc. Lots of services have a minimum letter requirement.

@nacin: I know that lots of services have restrictions in this field. But I think, that it would be great to let the service-provider let the choice for that. So yes, we should not approve my patch and create another one, which is able to filter all the posted values to filter. I'm going to write a new patch for that and will add it here.