[PATCH] Add getrandom implementation [BZ #17252]

[Roland McGrath]

> Based on the last discussion, I assumed that we had consensus that we'd 
> add wrappers for system calls which do not have merely niche applications.

I don't agree with that.  getrandom is a case that is justifiable on its
own terms as a new OS-independent GNU API, so make the case that way and
don't make the API (or any more of the implementation than needs to be)
Linux-specific.