ripd: retrofit the 'distance source' commands to the new northbound model

ripd: retrofit the 'distance source' commands to the new northbound model

The "distance (1-255) A.B.C.D/M [WORD]" command was modeled using a
YANG list, which makes it a little bit more complicated to convert to
the new northbound model.

The rip_distance_set() and rip_distance_unset() functions were removed
since they set/unset multiple configuration options at the same time. The
northbound callbacks need to set/unset configuration options individually.

When a distance list is created, use yang_dnode_set_entry() to store
a pointer in the configuration node, and retrieve this pointer in the
other callbacks using yang_dnode_get_entry().

The 'rip_distance' structure was moved to ripd.h so that it can be used
in the rip_northbound.c file.

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