Handle case collisions on case-insensitive systems

When running NixOps under Mac OS X, we need to be able to import store
paths built on Linux into the local Nix store. However, HFS+ is
usually case-insensitive, so if there are directories with file names
that differ only in case, then importing will fail.

The solution is to add a suffix ("~nix~case~hack~<integer>") to
colliding files. For instance, if we have a directory containing
xt_CONNMARK.h and xt_connmark.h, then the latter will be renamed to
"xt_connmark.h~nix~case~hack~1". If a store path is dumped as a NAR,
the suffixes are removed. Thus, importing and exporting via a
case-insensitive Nix store is round-tripping. So when NixOps calls
nix-copy-closure to copy the path to a Linux machine, you get the
original file names back.

Closes #119.

src/libstore/globals.cc

@@ -2,6 +2,7 @@
 
 #include "globals.hh"
 #include "util.hh"
+#include "archive.hh"
 
 #include <map>
 #include <algorithm>
@@ -150,6 +151,7 @@ void Settings::update()
     get(useSshSubstituter, "use-ssh-substituter");
     get(logServers, "log-servers");
     get(enableImportNative, "allow-unsafe-native-code-during-evaluation");
+    get(useCaseHack, "use-case-hack");
 
     string subs = getEnv("NIX_SUBSTITUTERS", "default");
     if (subs == "default") {