security: re-enable temporarily disabled security checks

security: re-enable temporarily disabled security checks

- Revert proc_find_pid_safe to simple version (remove disabled UID check)
- Revert shm.c comment to original NX flag message
- Remove commented-out SOCK_RAW privilege check in socket.c

These checks were temporarily disabled in commit 63566ad to investigate
test failures but were never re-enabled. With NX support properly
implemented in commit 5d72805, all checks can now be active.

Test: 119/119 PASS (SMP=4)