Enable IP_BOUND_IF on illumos and Solaris

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

src/sys/unix.rs

@@ -1834,6 +1834,8 @@ impl crate::Socket {
             target_os = "macos",
             target_os = "tvos",
             target_os = "watchos",
+            target_os = "illumos",
+            target_os = "solaris",
         )
     ))]
     pub fn bind_device_by_index_v4(&self, interface: Option<NonZeroU32>) -> io::Result<()> {
@@ -1859,6 +1861,8 @@ impl crate::Socket {
             target_os = "macos",
             target_os = "tvos",
             target_os = "watchos",
+            target_os = "illumos",
+            target_os = "solaris",
         )
     ))]
     pub fn bind_device_by_index_v6(&self, interface: Option<NonZeroU32>) -> io::Result<()> {
@@ -1879,6 +1883,8 @@ impl crate::Socket {
             target_os = "macos",
             target_os = "tvos",
             target_os = "watchos",
+            target_os = "illumos",
+            target_os = "solaris",
         )
     ))]
     pub fn device_index_v4(&self) -> io::Result<Option<NonZeroU32>> {
@@ -1900,6 +1906,8 @@ impl crate::Socket {
             target_os = "macos",
             target_os = "tvos",
             target_os = "watchos",
+            target_os = "illumos",
+            target_os = "solaris",
         )
     ))]
     pub fn device_index_v6(&self) -> io::Result<Option<NonZeroU32>> {