win32: dirs: fix potential buffer overflow when appending "portable"

We let the concatenation decide if there's enough room or not.

And use the Wide char calls where wide char pointers are used.

changed milestone to %4.0

added Component::Core Platform::Windows desktop Status::BackportCandidate labels

added MRStatus::Reviewable label

added MRStatus::Accepted label and removed MRStatus::Reviewable label

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added 36 commits

• d52e1be6...bef04581 - 35 commits from branch videolan:master
• 28241fad - win32: dirs: fix potential buffer overflow when appending "portable"

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enabled an automatic merge when all merge checks for 28241fad pass

merged