os: don't hold ForkLock across opens on Plan 9

If os.OpenFile holds ForkLock on files that block opens,
then threads that simultaneously try to do fork-exec will
get hung up (until the open succeeds). Blocked opens are
common enough on Plan 9 that protecting against fd leaks
into fork-execs means not being able to do fork-execs
properly in the general case. Thus, we forgo taking the
lock.

R=rsc, ality
CC=golang-dev
https://codereview.appspot.com/7235066

Committer: Russ Cox <rsc@golang.org>