nixos: rewrite the grsecurity module

The new module is specifically adapted to the NixOS Grsecurity/PaX
kernel.  The module declares the required kernel configurations and
so *should* be somewhat compatible with custom Grsecurity kernels.

The module exposes only a limited number of options, minimising the need
for user intervention beyond enabling the module. For experts,
Grsecurity/PaX behavior may be configured via `boot.kernelParams` and
`boot.kernel.sysctl`.

The module assumes the user knows what she's doing (esp. if she decides
to modify configuration values not directly exposed by the module).

Administration of Grsecurity's role based access control system is yet
to be implemented.