nix-store --serve: pass on settings.keepFailed from SSH store

When doing e.g.

    nix-build -A package --keep-failed --option \
      builders \
      'ssh://mfhydra?remote-store=/home/bosch/store x86_64-linux - 10 4 big-parallel'

this doesn't work properly because this build-setting is ignored.

I changed this behavior by passing the `settings.keepFailed` through the
serve-protocol to remote machines to make sure that I can introspect the
build-directory (which is particularly helpful when I have to look at a
`config.log` from a failed build for instance).

tests/failing.nix

@@ -0,0 +1,22 @@
+{ busybox }:
+with import ./config.nix;
+let
+
+  mkDerivation = args:
+    derivation ({
+      inherit system;
+      builder = busybox;
+      args = ["sh" "-e" args.builder or (builtins.toFile "builder-${args.name}.sh" "if [ -e .attrs.sh ]; then source .attrs.sh; fi; eval \"$buildCommand\"")];
+    } // removeAttrs args ["builder" "meta"])
+    // { meta = args.meta or {}; };
+in
+{
+
+  failing = mkDerivation {
+    name = "failing";
+    buildCommand = ''
+      echo foo > bar
+      exit 1
+    '';
+  };
+}