nix-store -l: Fetch build logs from the Internet

If a build log is not available locally, then ‘nix-store -l’ will now
try to download it from the servers listed in the ‘log-servers’ option
in nix.conf. For instance, if you have:

  log-servers = http://hydra.nixos.org/log

then it will try to get logs from http://hydra.nixos.org/log/<base
name of the store path>. So you can do things like:

  $ nix-store -l $(which xterm)

and get a log even if xterm wasn't built locally.

src/libstore/globals.hh

@@ -197,6 +197,9 @@ struct Settings {
     /* Whether to show a stack trace if Nix evaluation fails. */
     bool showTrace;
 
+    /* A list of URL prefixes that can return Nix build logs. */
+    Strings logServers;
+
 private:
     SettingsMap settings, overrides;