luci-proto-relay: propagate up state, do not forward netifd errors

luci-proto-relay: propagate up state, do not forward netifd errors

Due to historical reasons, the relayd daemon configuration resides in the
form of a fake "config interface" section in /etc/config/network without
actually registering a protocol handler.

This causes netifd to emit an "INVALID_PROTO" error for the interface
which is technically correct, but confusing for LuCI users.

This situation needs to be resolved upstream by either moving relayd
configuration out of the network config, or by converting the relayd
service into a proper protocol handler.

Until this happens, do not report any netifd errors since we're not
operating on an actual interface.

While we're at it, also propagate the protocol up state from the fake
virtual device to ensure that LuCI displays the relay bridge as "up"
when all relayed interfaces are up as well.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>
(cherry picked from commit aae187dedf726fe609ede8260f8f8343d843d046)