Make cgroup.{cc,hh} linux-only files

Forcing a conditional include, vs making the headers content
conditional, I think is more maintainable.

It is also how the other platform-specific headers (like
`namespaces.hh`) have been adapted.

src/libutil/linux/cgroup.hh

@@ -0,0 +1,28 @@
+#pragma once
+///@file
+
+#include <chrono>
+#include <optional>
+
+#include "types.hh"
+
+namespace nix {
+
+std::optional<Path> getCgroupFS();
+
+std::map<std::string, std::string> getCgroups(const Path & cgroupFile);
+
+struct CgroupStats
+{
+    std::optional<std::chrono::microseconds> cpuUser, cpuSystem;
+};
+
+/**
+ * Destroy the cgroup denoted by 'path'. The postcondition is that
+ * 'path' does not exist, and thus any processes in the cgroup have
+ * been killed. Also return statistics from the cgroup just before
+ * destruction.
+ */
+CgroupStats destroyCgroup(const Path & cgroup);
+
+}