Move /src to /subprojects

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

subprojects/libutil/signals.hh

@@ -0,0 +1,65 @@
+#pragma once
+///@file
+
+#include "types.hh"
+#include "error.hh"
+#include "logging.hh"
+
+#include <functional>
+
+namespace nix {
+
+/* User interruption. */
+
+/**
+ * @note Does nothing on Windows
+ */
+static inline void setInterrupted(bool isInterrupted);
+
+/**
+ * @note Does nothing on Windows
+ */
+static inline bool getInterrupted();
+
+/**
+ * @note Does nothing on Windows
+ */
+void setInterruptThrown();
+
+/**
+ * @note Does nothing on Windows
+ */
+inline void checkInterrupt();
+
+/**
+ * @note Never will happen on Windows
+ */
+MakeError(Interrupted, BaseError);
+
+
+struct InterruptCallback
+{
+    virtual ~InterruptCallback() { };
+};
+
+/**
+ * Register a function that gets called on SIGINT (in a non-signal
+ * context).
+ *
+ * @note Does nothing on Windows
+ */
+std::unique_ptr<InterruptCallback> createInterruptCallback(
+    std::function<void()> callback);
+
+/**
+ * A RAII class that causes the current thread to receive SIGUSR1 when
+ * the signal handler thread receives SIGINT. That is, this allows
+ * SIGINT to be multiplexed to multiple threads.
+ *
+ * @note Does nothing on Windows
+ */
+struct ReceiveInterrupts;
+
+}
+
+#include "signals-impl.hh"