vet: improve flag handling
Simplify the internal logic for flags controlling what to vet,
by introducing a map of flags that gathers them all together.
This change should simplify the process of adding further flags.

Add a test for untagged struct literals.
Delete a redundant test that was also in the wrong file.
Clean up some ERROR patterns that weren't working.

"make test" passes again.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/7305075