Enhance name duplication detection to account for homoglyphs

[shawnb61]

SMF's existing name duplication checks can be thwarted using homoglyphs.

For example, these two names are distinct & would both be allowed in SMF: "Mіau!" and "Miau!".

More discussion here:
https://www.simplemachines.org/community/index.php?topic=563837.0

Definition & examples of homoglyphs here:
https://en.wikipedia.org/wiki/Homoglyph

[Arantor]

The problem is how large the list is. How far down that list do you go?

[shawnb61]

Yep.  Still, worthy to have that discussion & consider the enhancement. 

I think with more sites using utf8 (esp. 2.1) this warrants consideration.

[Kindred]

Personally, I don't think that this is a large enough issue to waste development time doing it...  seriously, I have seen this sort of issue reported exactly twice in over a decade.

[Sesquipedalian]

It might be possible to create a mod to do this relatively reliably and without killing the server, but it won't become a standard feature of SMF any time soon.

Such a mod could use an approach similar to the one I used in 2.1's set_tld_regex() function, by periodically downloading and processing this official file to build an array of substitutions to normalize confusable characters and strings. This array would need to be stored in the database somewhere, and one would want to add a column to the members table that recorded each member's normalized username to use for comparisons.