fix(memory): provide better free memory approximation on old kernels

The approximation should include SReclaimable, and subtract Shmem. To
prevent the parsing code from ballooning in size, this commit also
refactors the parsing into a map.

include/modules/memory.hpp

@@ -17,8 +17,7 @@ class Memory : public ALabel {
   static inline const std::string data_dir_ = "/proc/meminfo";
   void                            parseMeminfo();
 
-  unsigned long memtotal_;
-  unsigned long memfree_;
+  std::unordered_map<std::string, unsigned long> meminfo_;
 
   util::SleeperThread thread_;
 };