RFE dhcrelay: add option to force giaddr

Problem

Some buggy dhcp clients (e.g. Solaris grub, which gets used e.g. for network install) use the address of the dhcp gateway (the host on which dhcrelay is running) to setup its default route instead of the announced default router (3) entry. Therefore it is not able to download the config/kernel/miniroot and network [emergency] boot does not work (unless the required server are in the same [V]LAN/subnet).

Solution

Add an option e.g. "-g <ip_address>" to force setting the gw address in packages sent back to the dhclient to the given <ip_address>.

Alternatives

None.

Funding its development

No funding is needed. We already developed a patch and use it for several years on our gpu cluster as well as department networks and can be considered stable. However, wondering how to submit it. Clone and add a PR or just attaching it as plain text?