spec: clarify short variable declaration corner cases

Fixes issue 4612.

R=rsc, iant, ken, r
CC=golang-dev
https://codereview.appspot.com/7076043
go/parser: better error message for declaration error

Fixes issue 4616.

R=rsc
CC=golang-dev
https://codereview.appspot.com/7069049
go/printer, gofmt: don't print unneeded parentheses around parameter types

Fixes issue 4624.

R=rsc
CC=golang-dev
https://codereview.appspot.com/7058052