Add MALLOC_DEBUG env var to debug applications using malloc

lib/libc/ansi/malloc-debug.h

@@ -0,0 +1,8 @@
+#include <minix/u64.h>
+#include <sys/types.h>
+
+/* malloc-debug.c */
+void *_dbg_malloc(size_t size);
+void *_dbg_realloc(void *oldp, size_t size);
+void _dbg_free(void *ptr);
+