nixos/security/misc: init

A module for security options that are too small to warrant their own module.

The impetus for adding this module is to make it more convenient to override
the behavior of the hardened profile wrt user namespaces.
Without a dedicated option for user namespaces, the user needs to
1) know which sysctl knob controls userns
2) know how large a value the sysctl knob needs to allow e.g.,
   Nix sandbox builds to work

In the future, other mitigations currently enabled by the hardened profile may
be promoted to options in this module.