Linux: individually show shared memory

Shmem: Total memory used by shared memory (shmem) and tmpfs

Source: https://www.kernel.org/doc/Documentation/filesystems/proc.txt

Closes: #556

ProcessList.h

@@ -68,6 +68,7 @@ typedef struct ProcessList_ {
    memory_t usedMem;
    memory_t buffersMem;
    memory_t cachedMem;
+   memory_t sharedMem;
    memory_t availableMem;
 
    memory_t totalSwap;