radvd >= 2.0 blocks router update processing

This is [pre-OSSA] Vulnerability in OpenStack Neutron (CVE-2014-8153)

Original bug: https://bugs.launchpad.net/neutron/+bug/1398779

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In radvd 2.0+, daemonization code was rewritten, switching from libdaemon's daemon_fork() to Linux daemon() call.

If no logging method (-m option) is passed to radvd, and the default logging method is used (which is L_STDERR_SYSLOG), then daemon() is called with (1, 1) arguments, meaning no chroot (fine) and not closing stderr (left there for logging) (not fine). So execute() call that spawns radvd and expects it to daemonize and return code never actually completes, blocked on stderr.

The fix is to pass e.g. -m syslog to radvd to make it close stderr and return.