[PATCH v2] Use execveat syscall in fexecve

[Florian Weimer]

* Joseph Myers:

> We have the ENOSYS code there and the subsequent fallback that will 
> eventually need cleaning up *anyway*.  Having the __ASSUME_* macro makes 
> it obvious in future exactly what changes should be made for the cleanup 
> and exactly when it is possible to do that cleanup.  I think having such 
> macros for any case with fallback code that can be cleaned up in future is 
> a good idea for that reason.

Agreed.  As long as there is an expectation that we will eventually
bump the minimum kernel requirement, __ASSUME_* macros are the way to
go.