Support mailto: URIs in DEP3 Forwarded values.

Bas Couwenbergrequested to merge
sebastic/udd:patches-forwarded-mailto into master

UDD erroneously complains about invalid Forwarded values [1].

Those patches were forwarded upstream via email as indicated with mailto: URIs, there are a few packages using this:

https://codesearch.debian.net/search?q=Forwarded%3A+mailto%3A+path%3Adebian%2Fpatches&literal=1&perpkg=1

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