Start factoring out Unix assumptions

This splits files and adds new identifiers in preperation for supporting
windows, but no Windows-specific code is actually added yet.

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

src/libstore/filetransfer.cc

@@ -1,5 +1,4 @@
 #include "filetransfer.hh"
-#include "namespaces.hh"
 #include "globals.hh"
 #include "store-api.hh"
 #include "s3.hh"
@@ -12,6 +11,10 @@
 #include <aws/core/client/ClientConfiguration.h>
 #endif
 
+#if __linux__
+# include "namespaces.hh"
+#endif
+
 #include <unistd.h>
 #include <fcntl.h>
 
@@ -568,7 +571,9 @@ struct curlFileTransfer : public FileTransfer
             stopWorkerThread();
         });
 
+        #if __linux__
         unshareFilesystem();
+        #endif
 
         std::map<CURL *, std::shared_ptr<TransferItem>> items;