Here is a new attempt at providing the getrandom system call.

I have retained the __getrandom symbol mangling.  The justification for 
that is that getrandom is a fairly common name.  Application code might 
use it for something else entirely and interpose their definition, so 
that libraries cannot rely on it doing the right thing.  I think the 
mangling is justified because it is hard to spot that getrandom is 
broken due to interposition.  As <sys/random.h> is a new header, the 
macro will be exposed to few applications.

I have made the system call wrapper a cancellation point.  (If we 
implement the simpler getentropy interface, it would not be a 
cancellation point.)

Thanks,
Florian