download: check available disk space before downloading

We don't check *while* the download happens, so it might
still be that we don't have enough space later

src/common/download.cpp

@@ -95,6 +95,14 @@ namespace tools
           {
             MINFO("Content-Length: " << length);
             content_length = length;
+            boost::filesystem::path path(control->path);
+            boost::filesystem::space_info si = boost::filesystem::space(path);
+            if (si.available < (size_t)content_length)
+            {
+              const uint64_t avail = (si.available + 1023) / 1024, needed = (content_length + 1023) / 1024;
+              MERROR("Not enough space to download " << needed << " kB to " << path << " (" << avail << " kB available)");
+              return false;
+            }
           }
           return true;
         }