syscall: ignore getwd errors when fixing working directory on Plan 9
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.