Move /src to /subprojects

This will facilitate breaking up Nix into multiple packages for each
component with Meson.

subprojects/libutil/linux/namespaces.hh

@@ -0,0 +1,35 @@
+#pragma once
+///@file
+
+#include <optional>
+
+#include "types.hh"
+
+namespace nix {
+
+/**
+ * Save the current mount namespace. Ignored if called more than
+ * once.
+ */
+void saveMountNamespace();
+
+/**
+ * Restore the mount namespace saved by saveMountNamespace(). Ignored
+ * if saveMountNamespace() was never called.
+ */
+void restoreMountNamespace();
+
+/**
+ * Cause this thread to try to not share any FS attributes with the main
+ * thread, because this causes setns() in restoreMountNamespace() to
+ * fail.
+ *
+ * This is best effort -- EPERM and ENOSYS failures are just ignored.
+ */
+void tryUnshareFilesystem();
+
+bool userNamespacesSupported();
+
+bool mountAndPidNamespacesSupported();
+
+}