Move /src to /subprojects

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

subprojects/libutil/current-process.hh

@@ -0,0 +1,38 @@
+#pragma once
+///@file
+
+#include <optional>
+
+#ifndef _WIN32
+# include <sys/resource.h>
+#endif
+
+#include "types.hh"
+
+namespace nix {
+
+/**
+ * If cgroups are active, attempt to calculate the number of CPUs available.
+ * If cgroups are unavailable or if cpu.max is set to "max", return 0.
+ */
+unsigned int getMaxCPU();
+
+/**
+ * Change the stack size.
+ */
+void setStackSize(size_t stackSize);
+
+/**
+ * Restore the original inherited Unix process context (such as signal
+ * masks, stack size).
+
+ * See unix::startSignalHandlerThread(), unix::saveSignalMask().
+ */
+void restoreProcessContext(bool restoreMounts = true);
+
+/**
+ * @return the path of the current executable.
+ */
+std::optional<Path> getSelfExe();
+
+}