grsecurity: discontinue support

Upstream has decided to make -testing patches private, effectively ceasing
free support for grsecurity/PaX [1].  Consequently, we can no longer
responsibly support grsecurity on NixOS.

This patch turns the kernel and patch expressions into build errors and
adds a warning to the manual, but retains most of the infrastructure, in
an effort to make the transition smoother.  For 17.09 all of it should
probably be pruned.

[1]: https://grsecurity.net/passing_the_baton.php