fmt: change evalutation of indexed arg to match docs

fmt: change evalutation of indexed arg to match docs
The old code put the index before the period in the precision;
it should be after so it's always before the star, as documented.
A little trickier to do in one pass but compensated for by more
tests and catching a couple of other error cases.

R=rsc
CC=golang-dev
https://golang.org/cl/9751044