ripngd: retrofit the 'offset-list' command to the new northbound model

ripngd: retrofit the 'offset-list' command to the new northbound model

Remove the ripng_offset_list_set() and ripng_offset_list_unset()
functions since they set/unset multiple configuration options at the
same time. The northbound callbacks need to set/unset configuration
options individually.

The frr-ripngd YANG module models the "offset-list" command using a
list keyed by the 'interface' and 'direction' leafs. One important
detail is that the IFNAME parameter is optional, and when it's not
present it means we want to match all interfaces. This is modeled
using an interface name of '*' since list keys are mandatory leafs
by definition in YANG.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>