Zabbix

.......PS. [ZBXNEXT-1554] code improvements and bug fixes:

fixed type of max_error_len;
fixed call of zbx_get_snmp_response_error(): sizeof(error) -> max_error_len;
fixed order of arguments in cache_put_snmp_index();
rewrote zbx_snmp_process_discovery() to accept one item;
minor code changes