net, internal/routebsd: move vendored x/net/route to internal

net, internal/routebsd: move vendored x/net/route to internal

This is a simple move of the contents of the vendored x/net/route
to internal/routebsd. I've also added some test files that
were not previously vendored.

This next CL will simplify the new internal/routebsd, removing the
code that is not needed by the new package.

This is a step toward simplifying the x/net/route package by
permitting it to import x/sys/unix.

Change-Id: I4d13df11fa9738cd68876b2ea456d03f82d8d64a
Reviewed-on: https://go-review.googlesource.com/c/go/+/637695
Commit-Queue: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Commit-Queue: Ian Lance Taylor <iant@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>