Reimplement connect-timeout

Fixes #1339.

src/libstore/download.cc

@@ -249,6 +249,8 @@ struct CurlDownloader : public Downloader
                 curl_easy_setopt(req, CURLOPT_SSL_VERIFYHOST, 0);
             }
 
+            curl_easy_setopt(req, CURLOPT_CONNECTTIMEOUT, settings.connectTimeout.get());
+
             /* If no file exist in the specified path, curl continues to work
                anyway as if netrc support was disabled. */
             curl_easy_setopt(req, CURLOPT_NETRC_FILE, settings.netrcFile.get().c_str());

src/libstore/globals.hh

@@ -270,6 +270,9 @@ public:
     struct CaseHackTag { };
     Setting<bool, CaseHackTag> useCaseHack{this, nix::useCaseHack, "use-case-hack",
         "Whether to enable a Darwin-specific hack for dealing with file name collisions."};
+
+    Setting<unsigned long> connectTimeout{this, 0, "connect-timeout",
+        "Timeout for connecting to servers during downloads. 0 means use curl's builtin default."};
 };