Added Vendor Class Identifier in DHCP offers and requests.

[kbenyous]

The Vendor Class Identifier is a constant string that identifies the kind of hardware that asks the DHCP server for a IP address. The ID is passed via the option 60 of the DHCP messages.

The ID is set to "WIZnetSE" constant.

From a DHCP server point of view, this ID is useful for giving specific configuration for Wiznet modules : IP address from different address pool, different gateway, longer lease, etc.

Reference documentation : https://datatracker.ietf.org/doc/html/rfc2132#section-9.13