Auto merge of #1497 - asomers:rlim_nlimits, r=gnzlbg

Deprecate RLIM_NLIMITS

This constant is not stable across OS versions, so it cannot be used in
any backwards- or forwards- compatible way.  It's typically used to size
arrays in the kernel and in debugging utilities that are closely tied to
the OS version.  Since libc is ignorant about OS versions, we shouldn't
even be defining it.