pppoe: T9356: do not fail commit when no Router Advertisement is received (#5504)

The Router Solicitation kicked off when an IPv6 autoconf PPPoE link comes up
is best effort - the peer may answer late or not at all. rdisc6(8) then blocks
for the duration of all retries and exits non-zero, which aborted the commit:

  FileNotFoundError: [Errno 2] failed to run command: rdisc6 --single --retry 3 pppoe1000
  returned: Soliciting ff02::2 (ff02::2) on pppoe1000...
  Timed out.
  No response.
  exit code: 2

Run rdisc6(8) detached in a transient systemd unit, so neither the delay nor
the exit code reaches the commit. The outcome is logged to the journal.

0 files changed, 0 insertions, 0 deletions