Fix userland tools that don't check the format of routing socket
messages before accessing message fields that may not be present,
removing dead/duplicate/misleading code along the way.

Document the message format for each routing socket message in
route.h.

Fix a bug in usr.bin/netstat introduced in r287351 that resulted in
pointer computation with essentially random 16-bit offsets and
dereferencing of the results.

Reviewed by: ae
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D10330