Enable IP_BOUND_IF on illumos and Solaris (#565)

The `IP_BOUND_IF` socket option, which is wrapped by the
`Socket::bind_device_by_index_{v4,v6}` and
`Socket::device_index_{v4,v6}` is available on SunOS-like systems, such
as illumos and Solaris, as well as macOS-like systems. However, these
APIs are currently cfg-flagged to only be available on macOS-like
systems.

This commit changes the cfg attributes to also enable these APIs on
illumos and Solaris.

Fixes #560
Backport of 5c1f3b0

Author: hawkw