ovpn: keep carrier always on for MP interfaces

ovpn: keep carrier always on for MP interfaces

An ovpn interface configured in MP mode will keep carrier always
on and let the user decide when to bring it administratively up and
down.

This way a MP node (i.e. a server) will keep its interface always
up and running, even when no peer is connected.

Signed-off-by: Antonio Quartulli <antonio@openvpn.net>
Link: https://patch.msgid.link/20250415-b4-ovpn-v26-4-577f6097b964@openvpn.net
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>