Auto merge of #1260 - nbaksalyar:add-missing-solaris-fns, r=gnzlbg
Auto merge of #1260 - nbaksalyar:add-missing-solaris-fns, r=gnzlbg

Add missing group entry functions for Solaris/Illumos

This commit adds definitions for `setgrent`, `endgrent`, and `getgrent` which were missing (but available for FreeBSD and other *nix systems).

rust-lang/libc,rust-lang-nursery/libc,rust-lang/libc,rust-lang-nursery/libc,rust-lang-nursery/libc,rust-lang/libc