[hurd,commited] hurd: do not check Mach and Hurd headers

[Samuel Thibault]

Joseph Myers, on sam. 03 mars 2018 22:53:23 +0000, wrote:
> it should always be possible to write a header in a way that 
> doesn't have such a requirement.

I'm almost there, the only remaining issue is that hurd/hurd/signal.h
uses struct sigaction.  I guess I should move existing definitions to a
struct_sigaction.h header?

Samuel