Misc Windows fixes

1. Fix build by making the legacy SSH Storey's secret `logFD` setting
   not a setting on Windows. (It doesn't make sense to specify `void *`
   handles by integer cross-proccess, I don't think.)

2. Move some files that don't need to be Unix-only anymore back to their
   original locations.

src/libstore/builtins/unpack-channel.cc

@@ -0,0 +1,33 @@
+#include "builtins.hh"
+#include "tarfile.hh"
+
+namespace nix {
+
+void builtinUnpackChannel(
+    const BasicDerivation & drv,
+    const std::map<std::string, Path> & outputs)
+{
+    auto getAttr = [&](const std::string & name) {
+        auto i = drv.env.find(name);
+        if (i == drv.env.end()) throw Error("attribute '%s' missing", name);
+        return i->second;
+    };
+
+    auto out = outputs.at("out");
+    auto channelName = getAttr("channelName");
+    auto src = getAttr("src");
+
+    createDirs(out);
+
+    unpackTarfile(src, out);
+
+    auto entries = std::filesystem::directory_iterator{out};
+    auto fileName = entries->path().string();
+    auto fileCount = std::distance(std::filesystem::begin(entries), std::filesystem::end(entries));
+
+    if (fileCount != 1)
+        throw Error("channel tarball '%s' contains more than one file", src);
+    std::filesystem::rename(fileName, (out + "/" + channelName));
+}
+
+}