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.

tests/case-hack.sh

@@ -0,0 +1,19 @@
+source common.sh
+
+clearStore
+
+rm -rf $TEST_ROOT/case
+
+opts="--option use-case-hack true"
+
+# Check whether restoring and dumping a NAR that contains case
+# collisions is round-tripping, even on a case-insensitive system.
+nix-store $opts  --restore $TEST_ROOT/case < case.nar
+nix-store $opts --dump $TEST_ROOT/case > $TEST_ROOT/case.nar
+cmp case.nar $TEST_ROOT/case.nar
+[ "$(nix-hash $opts --type sha256 $TEST_ROOT/case)" = "$(nix-hash --flat --type sha256 case.nar)" ]
+
+# Check whether we detect true collisions (e.g. those remaining after
+# removal of the suffix).
+touch "$TEST_ROOT/case/xt_CONNMARK.h~nix~case~hack~3"
+! nix-store $opts --dump $TEST_ROOT/case > /dev/null