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.cc

@@ -147,6 +147,7 @@ void Settings::update()
     get(envKeepDerivations, "env-keep-derivations");
     get(sshSubstituterHosts, "ssh-substituter-hosts");
     get(useSshSubstituter, "use-ssh-substituter");
+    get(logServers, "log-servers");
 
     string subs = getEnv("NIX_SUBSTITUTERS", "default");
     if (subs == "default") {