One first round of low-hanging fruit, excluding anything unclear.

Also fixed a bug where, if a contributor had different emails under
different CLAs, only the first one was added to AUTHORS.

Updates #12042

Change-Id: Id7b06c885d74b4718ef2d74d149513a7c0f40c91
Reviewed-on: https://go-review.googlesource.com/126215Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>