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/libutil/archive.hh

@@ -28,7 +28,7 @@ namespace nix {
 
    where:
 
-     attrs(as) = concat(map(attr, as)) + encN(0) 
+     attrs(as) = concat(map(attr, as)) + encN(0)
      attrs((a, b)) = encS(a) + encS(b)
 
      encS(s) = encN(len(s)) + s + (padding until next 64-bit boundary)
@@ -58,7 +58,7 @@ void dumpPath(const Path & path, Sink & sink,
 struct ParseSink
 {
     virtual void createDirectory(const Path & path) { };
-    
+
     virtual void createRegularFile(const Path & path) { };
     virtual void isExecutable() { };
     virtual void preallocateContents(unsigned long long size) { };
@@ -66,10 +66,14 @@ struct ParseSink
 
     virtual void createSymlink(const Path & path, const string & target) { };
 };
-    
+
 void parseDump(ParseSink & sink, Source & source);
 
 void restorePath(const Path & path, Source & source);
 
- 
+
+// FIXME: global variables are bad m'kay.
+extern bool useCaseHack;
+
+
 }