This changes fixes two issues with regard to handling routing messages
as follows:
- Misparsing on platforms (such as FreeBSD) supporting multiple
  architectures in the same kernel (kern.supported_archs="amd64 i386")
- Misparsing with unimplemented messages such as route, interface
  address state notifications

To fix those issues, this change implements all the required socket
address parsers, adds a processor architecture identifying function to
FreeBSD and tests.

Fixes #9707.
Fixes #8203.

Change-Id: I7ed7b4a0b6f10f54b29edc681a2f35603f2d8d45
Reviewed-on: https://go-review.googlesource.com/4330Reviewed-by: Ian Lance Taylor <iant@golang.org>