Zabbix

...G...... [ZBX-11720] fixed memory leak which breaks vfs.fs.size, vfs.fs.inode and vfs.dir.size items if compiled with LeakSanitizer

This change:
   - fixes agent memory leak (on Unix platforms),
   - on Unix platforms modifies zbx_execute_threaded_metric() to show child process exit code in "Data gathering process terminated..." error messages,
   - fixes a function prototype.