[tailscale1.16] syscall: 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.

Fixes golang#45767

(cherry picked from golang.org/cl/313769)

Change-Id: If190ace0e0213207fdaf6eeb79a5543ef18456de
Reviewed-on: https://go-review.googlesource.com/c/go/+/313769
Trust: Tobias Klauser <[email protected]>
Run-TryBot: Tobias Klauser <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
TryBot-Result: Go Bot <[email protected]>

Author: tklauser

src/syscall/syscall_linux.go

@@ -550,7 +550,11 @@ func anyToSockaddr(rsa *RawSockaddrAny) (Sockaddr, error) {
 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
 	}