fmt: make the %#v verb a special flag
The %#v verb is special: it says all values below need to print as %#v.
However, for some situations the # flag has other meanings and this
causes some issues, particularly in how Formatters work. Since %#v
dominates all formatting, translate it into actual state of the formatter
and decouple it from the # flag itself within the calculations (although
it must be restored when methods are doing the work.)
The result is cleaner code and correct handling of # for Formatters.
TODO: Apply the same thinking to the + flag in a followup CL.

Also, the wasString return value in handleMethods is always false,
so eliminate it.

Update issue 8835

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://codereview.appspot.com/146650043