all: vet output for core is noisy

[robpike]

Find a way to run vet regularly on the official go tree(s) and make it quiet.

This will require some scripting and cleanups.

[gopherbot]

CL https://golang.org/cl/19824 mentions this issue.

[gopherbot]

CL https://golang.org/cl/20002 mentions this issue.

[gopherbot]

CL https://golang.org/cl/20684 mentions this issue.

[gopherbot]

CL https://golang.org/cl/20685 mentions this issue.

[gopherbot]

CL https://golang.org/cl/22091 mentions this issue.

[gopherbot]

CL https://golang.org/cl/22318 mentions this issue.