GOSSAFUNC=foo had previously only done printing for the
single function foo, and didn't quite clean up after itself
properly. Changes ensures that Config.HTML != nil iff
GOSSAFUNC==name-of-current-function.

Change-Id: I255e2902dfc64f715d93225f0d29d9525c06f764
Reviewed-on: https://go-review.googlesource.com/29250
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>