* Get rid of the substitutes database table (NIX-47). Instead, if we

need any info on substitutable paths, we just call the substituters
  (such as download-using-manifests.pl) directly.  This means that
  it's no longer necessary for nix-pull to register substitutes or for
  nix-channel to clear them, which makes those operations much faster
  (NIX-95).  Also, we don't have to worry about keeping nix-pull
  manifests (in /nix/var/nix/manifests) and the database in sync with
  each other.

  The downside is that there is some overhead in calling an external
  program to get the substitutes info.  For instance, "nix-env -qas"
  takes a bit longer.

  Abolishing the substitutes table also makes the logic in
  local-store.cc simpler, as we don't need to store info for invalid
  paths.  On the downside, you cannot do things like "nix-store -qR"
  on a substitutable but invalid path (but nobody did that anyway).

* Never catch interrupts (the Interrupted exception).

src/libstore/globals.cc

@@ -25,6 +25,7 @@ unsigned int maxBuildJobs = 1;
 bool readOnlyMode = false;
 string thisSystem = "unset";
 unsigned int maxSilentTime = 0;
+Paths substituters;
 
 
 static bool settingsRead = false;