Don't know why, but git deleted the previous version of this change.
This is the same change as https://go-review.googlesource.com/11884,
which I will now abandon, with a couple of fixes.

Almost all done now. Could use help with the TODOs.

Major missing piece is the trace command. Vendoring
section is also weak, but it's also undocumented elsewhere.

Change-Id: I5d8556b23aa6628eb7bf0e330d4dd8d4ac2157c5
Reviewed-on: https://go-review.googlesource.com/11887Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>