go/parser: report illegal label declarations at ':' rather than guessing the start

go/parser: report illegal label declarations at ':' rather than guessing the start

Also:
- Add parser.SpuriousError flag. If set, the parser reports all (including
  spurious) errors rather then at most one error per line.
- Add -e flag to gofmt and gotype: If set, gofmt and gotype report all
  (including spurious) errors rather than at most one error per line.
- Updated the respective documentation.

Fixes #2088.

R=rsc
CC=golang-dev
https://golang.org/cl/4803047