libutil: make AutoCloseFD a better resource

add a reset() method to close the wrapped fd instead of assigning magic
constants. also make the from-fd constructor explicit so you can't
accidentally assign the *wrong* magic constant, or even an unrelated
integer that also just happens to be an fd by pure chance.

Change-Id: I51311b0f6e040240886b5103d39d1794a6acc325

src/libstore/binary-cache-store.cc

@@ -124,7 +124,7 @@ void BinaryCacheStore::writeNarInfo(ref<NarInfo> narInfo)
 
 AutoCloseFD openFile(const Path & path)
 {
-    auto fd = open(path.c_str(), O_RDONLY | O_CLOEXEC);
+    AutoCloseFD fd{open(path.c_str(), O_RDONLY | O_CLOEXEC)};
     if (!fd)
         throw SysError("opening file '%1%'", path);
     return fd;