os: use poll.fdMutex for Plan 9 files

os: use poll.fdMutex for Plan 9 files

This permits us to safely support concurrent access to files on Plan 9.
Concurrent access was already safe on other systems.

This does introduce a change: if one goroutine calls a blocking read
on a pipe, and another goroutine closes the pipe, then before this CL
the close would occur. Now the close will be delayed until the blocking
read completes.

Also add tests that concurrent I/O and Close on a pipe are OK.

For #50436
For #56043

Change-Id: I969c869ea3b8c5c2f2ef319e441a56a3c64e7bf5
Reviewed-on: https://go-review.googlesource.com/c/go/+/438347
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: David du Colombier <0intro@gmail.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>