kernel: disable accept_ra by default

Our commands setting accept_ra to 0 on all interfaces got lost in the
transition to procd. This remained unnoticed for a long time, as we also
enable forwarding on all interfaces, which prevents RA handling by default.

Restore the commands, while also fixing a possible race condition in the
old version.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>