Unsynchronized open-mode bookkeeping in cnopen/cnclose (D_MPSAFE)

Summary

cn_ops flagged D_MPSAFE(:71) but cnopen(:368-386)/cnclose(:415-436) read-modify-write cn_is_open/cn_phys_is_open/openmode/openflag with no token. Concurrent open/close desyncs bookkeeping -> close-while-in-use on physical console device. Root-only (SYSCAP_RESTRICTEDROOT :345 + 0600 node).