lib9: declare __fixargv0 before use in flag.c

The Plan 9 compilers complain about not
having type information for the function,
which sets off type signature problems
during the linking stage.

R=rsc, ality, iant
CC=golang-dev
https://codereview.appspot.com/7058054

Committer: Russ Cox <rsc@golang.org>
cmd/api: fix type scrubbing

It wasn't removing names from func parameters for func types,
and it was handling "a, b string" as "string", not "string, string".

Fixes issue 4688

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/7181051