zmalloc_get_private_dirty() function added (Linux only).

For non Linux systmes it just returns 0.

This function is useful to estimate copy-on-write because of childs
saving stuff on disk.

src/zmalloc.h

@@ -75,6 +75,7 @@ void zmalloc_enable_thread_safeness(void);
 void zmalloc_set_oom_handler(void (*oom_handler)(size_t));
 float zmalloc_get_fragmentation_ratio(void);
 size_t zmalloc_get_rss(void);
+size_t zmalloc_get_private_dirty(void);
 void zlibc_free(void *ptr);
 
 #ifndef HAVE_MALLOC_SIZE