Add a section to chapter 2 on system call interposition, ASLR and related mitigations

[g-kouv]

On #164, @lucic71 mentioned pinsyscall(2) from OpenBSD as an additional mitigation against code reuse attacks.

It'd be worthwhile to add a section describing mitigations beyond CFI that aren't just compiler-based, like the above mentioned pinsyscall, Linux's seccomp (and similar features in other systems, system call interposition more generally), a more detailed view of ASLR and so on. We can track ideas for this section here.