package/radvd: improve startup script

package/radvd: improve startup script

- Add start, stop and restart/reload options, following the logic used
  in other init scripts (e.g. S01syslogd).

- Do nothing if /etc/radvd.conf does not exist instead of printing an
  error message. It is valid to install radvd without a configuration
  file. The daemon may be started later by another service with a
  configuration created at run-time.

- Print an error message if the kernel does not support IPv6 forwarding,
  which is required by radvd.

Signed-off-by: Carlos Santos <unixmania@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>