Move route-specific ddb commands to route/route_ddb.c

Currently functionality resides in rtsock.c, which is a controlling
interface, partially external to the routing subsystem.
Additionally, DDB-supporting functionality is > 100SLOC, which deserves
a separate file.

Given that, move this functionality to a newly-created net/route/ subdir.

Reviewed by: cem
Differential Revision: https://reviews.freebsd.org/D24561