Add new special value "none" to ipv4.dhcp-client-id to allow not to send any DHCP Client-ID (Option 61), instead of sending a default generated one.
Sending a client-id is not mandatory according to RFC2131. It is mandatory according to RFC4361 that superseedes it.
Some weird DHCP servers conforming RFC2131 can get confused and break existing DHCP leases if they start receiving a client-id when it was not being previously received. Users that were using other DHCP client like dhclient, but want to use NetworkManager's internal DHCP client, can suffer this problem.
Add "none" as accepted value in ipv4.dhcp-client-id to specify that client-id must not be sent. Note that this is generally not recommended unless it's explicitly needed for some reason like the explained above.
Client-id is mandatory in DHCPv6.
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