pbrd: change the "set nexthop" command to accept interface nexthops

pbrd: change the "set nexthop" command to accept interface nexthops

In addition to nexthop groups, pbrd also supports the "set nexthop"
command to specify the nexthop of a PBR map. This adds convenience
when multiple nexthops aren't necessary.

Change this command to support interface nexthops (without IP
addresses) like nexthop groups do. At the end of the command, call
pbr_nht_nexthop_interface_update() otherwise the interface nexthop
won't be validated until we receive an interface up/down notification
from zebra through the zapi protocol.

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