[Issue 21057] getopt, function setter for a long option receive the key, not the value

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21057

--- Comment #1 from Basile-z <b2.temp at gmx.com> ---
This is blocking some work of me as using a setter is the only* way to
translate a string to a enum member that is a D keyword + underscore ;)

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