The process cmd, done and exitErr fields were written by Start/startCommand and
the waitForCommand goroutine while IsRunning/GetErr/GetResult/Stop read them
concurrently from other goroutines (the status endpoint and the check-xray
job) — a data race. Guard them with a RWMutex: writers take the write lock;
readers snapshot under the read lock and run any blocking syscall
(Wait/Signal/Kill) on the local copy without holding it. IsRunning now uses the
done channel as the exit signal instead of reading cmd.ProcessState, which
races with cmd.Wait. Adds a -race regression test.
n0ctal