In 4.x, it was lib/libmsg/addrutil.cpp. (They felt the need to convert it
to C++ for no adequately explainable reason, and to add some dependencies
on the rest of libmsg.)
Since libmime needs this, and libmsg isn't being built, I converted it
back to C, removed the libmsg dep, and put a copy of this file here.
Someday, something more sensible should be done. Like deleting the copy
in libmsg, perhaps.