nhrpd: shortcut routes installed with nexthop.

nhrpd: shortcut routes installed with nexthop.

Previously, when a shortcut entry was created, its associated route was
created on system, with no nexthop, only gre device. eg:

[..]
N>* 192.168.2.0/24 [10/0] is directly connected, gre1, 00:01:04           <--- can not be resolved

[..]
Type     Prefix                   Via                      Identity
dynamic  192.168.2.0/24           10.255.255.2             				<---- correct

This situation was forcing neighbor resolution on the first outgoing packet matching the route entry. for instance 192.168.2.1 could not be resolved at link layer, and was going to fail. Instead, nhrp nexthop should have been used.
This is what this commit intends to do, that is to say that when a
shortcut is installed by nhrp, the associated nexthop entry is used.

[..]
N>* 192.168.2.0/24 [10/0] via 10.255.255.2, gre1 onlink, 00:00:31

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>