unmapping of freed allocations without disabling chunk randomisation
like the "Freeguard" ('F') option does. Make security 'S' option
use 'U' and not 'F'.
Rationale: guarding with no chunk randomisation is great for debugging
use-after-free, but chunk randomisation offers better defence against
"heap feng shui" style attacks that depend on carefully constructing a
particular heap layout so we should leave this enabled when requesting
security options.