pumpChild's PTY.Read raced Session.Shutdown's PTY.Close on the master
field (Close set it nil while Read read it). Benign at process exit on
main, but the daemon now runs Shutdown routinely (daemon stop). Guard
the field with a mutex, capturing the fd under the lock and doing the
blocking I/O outside it so Close still unblocks an in-flight Read.
Caught under: go test -race -run 'Daemon|NetClient|Owner' -count=5.
