* Unify the treatment of sources copied to the store, and recursive

SHA-256 outputs of fixed-output derivations.  I.e. they now produce
  the same store path:

  $ nix-store --add x
  /nix/store/j2fq9qxvvxgqymvpszhs773ncci45xsj-x

  $ nix-store --add-fixed --recursive sha256 x
  /nix/store/j2fq9qxvvxgqymvpszhs773ncci45xsj-x

  the latter being the same as the path that a derivation

    derivation {
      name = "x";
      outputHashAlgo = "sha256";
      outputHashMode = "recursive";
      outputHash = "...";
      ...
    };

  produces.

  This does change the output path for such fixed-output derivations.
  Fortunately they are quite rare.  The most common use is fetchsvn
  calls with SHA-256 hashes.  (There are a handful of those is
  Nixpkgs, mostly unstable development packages.)
  
* Documented the computation of store paths (in store-api.cc).

src/nix-store/nix-store.cc

@@ -129,7 +129,7 @@ static void opAddFixed(Strings opFlags, Strings opArgs)
     opArgs.pop_front();
 
     for (Strings::iterator i = opArgs.begin(); i != opArgs.end(); ++i)
-        cout << format("%1%\n") % store->addToStore(*i, true, recursive, hashAlgo);
+        cout << format("%1%\n") % store->addToStore(*i, recursive, hashAlgo);
 }
 
 
@@ -151,6 +151,9 @@ static void opPrintFixedPath(Strings opFlags, Strings opArgs)
         if (*i == "--recursive") recursive = true;
         else throw UsageError(format("unknown flag `%1%'") % *i);
 
+    if (opArgs.size() != 3)
+        throw UsageError(format("`--print-fixed-path' requires three arguments"));
+    
     Strings::iterator i = opArgs.begin();
     string hashAlgo = *i++;
     string hash = *i++;