In Plan 9, goroutines can run in different processes,
which don't share their working directory. However,
Go expects the working directory to be program-wide.

We use a Fixwd function to fix the working directory
before calling system calls which depend on the
working directory.

In fixwdLocked, the working directory is not fixed
when getwd returns an error. However, an error can
happen is some cases, notably when the directory
has been previously removed in another process.

Fixes #10422.

Change-Id: Ie0c36f97c4b5ebe27ff0ead360987c5b35f825e4
Reviewed-on: https://go-review.googlesource.com/8800Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>