grsecurity module: disable EFI runtime services by default

Enabling EFI runtime services provides a venue for injecting code into
the kernel.

When grsecurity is enabled, we close this by default by disabling access
to EFI runtime services.  The upshot of this is that
/sys/firmware/efi/efivars will be unavailable by default (and attempts
to mount it will fail).

This is not strictly a grsecurity related option, it could be made into
a general option, but it seems to be of particular interest to
grsecurity users (for non-grsecurity users, there are other, more
immediate kernel injection attack dangers to contend with anyway).