grsecurity: support disabling TCP simultaneous connect

Defaults to OFF because disabling TCP simultaneous connect breaks some
legitimate use cases, notably WebRTC [1], but it's nice to provide the
option for deployments where those features are unneeded anyway.

This is an alternative to https://github.com/NixOS/nixpkgs/pull/4937

[1]: http://article.gmane.org/gmane.linux.documentation/9425