add acct, accept4, setns, and dup3 syscalls (linux extensions)

based on patch by Justin Cormack

src/linux/accept4.c

@@ -0,0 +1,9 @@
+#define _GNU_SOURCE
+#include <sys/socket.h>
+#include "syscall.h"
+#include "libc.h"
+
+int accept4(int fd, struct sockaddr *restrict addr, socklen_t *restrict len, int flg)
+{
+	return socketcall_cp(accept4, fd, addr, len, flg, 0, 0);
+}

src/linux/dup3.c

@@ -0,0 +1,10 @@
+#define _GNU_SOURCE
+#include <unistd.h>
+#include <errno.h>
+#include "syscall.h"
+
+int dup3(int old, int new, int flags) {
+        int r;
+        while ((r=__syscall(SYS_dup3, old, new, flags))==-EBUSY);
+        return __syscall_ret(r);
+}

src/linux/setns.c

@@ -0,0 +1,9 @@
+#define _GNU_SOURCE
+#include <sched.h>
+#include "syscall.h"
+#include "libc.h"
+
+int setns(int fd, int nstype)
+{
+        return syscall(SYS_setns, fd, nstype);
+}