RFC 2131 4.3.1 says lease time MUST be included in OFFER and ACK
so there is no need to request it.
define 28 request ipaddress broadcast_address
define 33 request array ipaddress static_routes
define 50 ipaddress dhcp_requested_address
-define 51 request uint32 dhcp_lease_time
+define 51 uint32 dhcp_lease_time
define 52 byte dhcp_option_overload
define 53 byte dhcp_message_type
define 54 ipaddress dhcp_server_identifier
define 48 array ipaddress font_servers
define 49 array ipaddress x_display_manager
define 50 ipaddress dhcp_requested_address
-define 51 request uint32 dhcp_lease_time
+define 51 uint32 dhcp_lease_time
define 52 byte dhcp_option_overload
define 53 byte dhcp_message_type
define 54 ipaddress dhcp_server_identifier