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

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

Remove the rip_offset_list_set() and rip_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-ripd 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 key lists are mandatory by definition in YANG.

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