Hash secret only 32-bit + sc_flags 8-bit near exhaustion: hardening gaps

Summary

hash_secret u_int(32-bit)(:157,:336) trivial XOR fold bucket hash. sc_flags u_int8_t(:521) SCF_* uses 7/8 bits(:522-528) no room for new flags. Reduces cost of hash-flood weakness(DF-0485). Fix: widen key to 128-bit keyed hash, widen sc_flags to u_int32_t.