internal/poll, net, syscall: use accept4 on illumos
Illumos supports the accept4 syscall, use it in internal/poll.accept
like on other platforms.
Add Accept4 to package syscall despite the package being frozen. The
other option would have been to add this to internal/syscall/unix, but
adding it to syscall avoids duplicating a lot of code in internal/poll
and net/internal/socktest. Also, all other platforms supporting the
accept4 syscall already export Accept4.