Switch Solaris-derivatives away from listen_unix (#6021)

Solaris 10 and Illumos are missing SO_REUSEPORT. Treat them more like
Windows (i.e. use the listener pool).

listen.go

@@ -12,7 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-//go:build !unix
+//go:build !unix || solaris
 
 package caddy
 

listen_unix.go

@@ -15,7 +15,7 @@
 // Even though the filename ends in _unix.go, we still have to specify the
 // build constraint here, because the filename convention only works for
 // literal GOOS values, and "unix" is a shortcut unique to build tags.
-//go:build unix
+//go:build unix && !solaris
 
 package caddy
 

listen_unix_setopt.go

@@ -1,4 +1,4 @@
-//go:build unix && !freebsd
+//go:build unix && !freebsd && !solaris
 
 package caddy