util: use stronger-guarantee rename method

Use std::filesystem::rename() instead of std::rename(). We rely on the
destination to be overwritten if it exists, but std::rename()'s behavior
is implementation-defined in this case.

src/util/system.h

@@ -69,7 +69,13 @@ void DirectoryCommit(const fs::path &dirname);
 bool TruncateFile(FILE *file, unsigned int length);
 int RaiseFileDescriptorLimit(int nMinFD);
 void AllocateFileRange(FILE *file, unsigned int offset, unsigned int length);
+
+/**
+ * Rename src to dest.
+ * @return true if the rename was successful.
+ */
 [[nodiscard]] bool RenameOver(fs::path src, fs::path dest);
+
 bool LockDirectory(const fs::path& directory, const std::string lockfile_name, bool probe_only=false);
 void UnlockDirectory(const fs::path& directory, const std::string& lockfile_name);
 bool DirIsWritable(const fs::path& directory);