grsecurity: revamp nixos kernel config

Cleanup:
- Restructure & add some commentary
- Remove redundant option specs given the auto config
  constraints (some are left in for documentation purposes)

Changes:
- GRKERNSEC_CONFIG_VIRT_HOST -> GUEST
  The former deselects paravirtualization and friends
- PAX_LATENT_ENTROPY n -> y (implied by auto)
- GRKERNSEC_ACL_HIDEKERN y -> n
  Possibly useless with redistribution