security: Round 4.2 SHM permissions (K14) - NX deferred (K24)

security: Round 4.2 SHM permissions (K14) - NX deferred (K24)

K14: SHM permission model
- Added uid, gid, mode fields to struct shm_segment
- Initialize uid/gid from current_process on shm_get
- Default mode = 0600 (rw-------)
- shm_at checks: only owner or root can attach

K24: NX flag deferred
- NX flag causes SIGSEGV because IA32_EFER.NXE MSR not enabled
- NX enforcement deferred until A01 (NX MSR enablement) is implemented

Tests: 119/119 PASS (smoke test, SMP=4)