unix: on linux use accept4 in Accept, fall back to accept

Android seems to block the accept syscall in newer versions. Use accept4
instead on kernel versions that support it (Linux 2.6.28 and newer) and
fall back to accept on ENOSYS.

Updates golang/go#45767

Change-Id: If557eaaaa0b69112bbe66ed820fbb382afb53b04
Reviewed-on: https://go-review.googlesource.com/c/sys/+/313690
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
diff --git a/unix/syscall_linux.go b/unix/syscall_linux.go
index 4263953..2dd7c8e 100644
--- a/unix/syscall_linux.go
+++ b/unix/syscall_linux.go
@@ -1151,7 +1151,11 @@
 func Accept(fd int) (nfd int, sa Sockaddr, err error) {
 	var rsa RawSockaddrAny
 	var len _Socklen = SizeofSockaddrAny
-	nfd, err = accept(fd, &rsa, &len)
+	// Try accept4 first for Android, then try accept for kernel older than 2.6.28
+	nfd, err = accept4(fd, &rsa, &len, 0)
+	if err == ENOSYS {
+		nfd, err = accept(fd, &rsa, &len)
+	}
 	if err != nil {
 		return
 	}