Minimum Linux kernel version for glibc 2.17

[Roland McGrath]

I think 2.6.16 is fine.  I concur with Mike's comments that
kernel-features.h macros should still test for the actual kernel
versions in macros that exist there.  I certainly think it's fine to
deconditionalize the actual code when that makes it smaller/cleaner.
But I think it would be wise that at least for any case that did not
exist in the first 2.6 kernel, we leave behind a comment saying in
which kernel version (we believe) a syscall or feature first appeared.


Thanks,
Roland