sftp: fix data race when using --sftp-ssh

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When the sftp backend uses an external ssh program (--sftp-ssh), one
goroutine waits for the ssh program to finish by calling cmd.Wait().
cmd.Wait() sets cmd.ProcessState when the program ends.

At the same time, CanReuse() calls exited(), which reads
cmd.ProcessState without any lock. This is a data race and the Go
race detector reports it. It happens in normal use, for example when
a connection is put back in the pool after a stat.

The race only decides whether a pooled connection can be reused, so
it is unlikely to cause problems in practice, but it makes programs
that use the sftp backend fail when they are tested with -race.

This fix sets an atomic.Bool after cmd.Wait() returns and makes
exited() read that instead of cmd.ProcessState.

It also adds a test which calls exited() while Wait() is running. It
fails with -race before this fix and passes after it.

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