Move net/route/shared.h definitions to net/route/route_var.h.

No functional changes.

net/route/shared.h was created in the inital phases of nexthop conversion.
It was intended to serve the same purpose as route_var.h - share definitions
of functions and structures between the routing subsystem components. At
that time route_var.h was included by many files external to the routing
subsystem, which largerly defeats its purpose.

As currently this is not the case anymore and amount of route_var.h includes
is roughly the same as shared.h, retire the latter in favour of the former.