Add a "fork" of musl as //fusl.

fusl/src/network/accept4.c

+#define _GNU_SOURCE
+#include <sys/socket.h>
+#include <errno.h>
+#include <fcntl.h>
+#include "syscall.h"
+#include "libc.h"
+
+int accept4(int fd, struct sockaddr *restrict addr, socklen_t *restrict len, int flg)
+{
+        if (!flg) return accept(fd, addr, len);
+        int ret = socketcall_cp(accept4, fd, addr, len, flg, 0, 0);
+        if (ret>=0 || (errno != ENOSYS && errno != EINVAL)) return ret;
+        ret = accept(fd, addr, len);
+        if (ret<0) return ret;
+        if (flg & SOCK_CLOEXEC)
+                __syscall(SYS_fcntl, ret, F_SETFD, FD_CLOEXEC);
+        if (flg & SOCK_NONBLOCK)
+                __syscall(SYS_fcntl, ret, F_SETFL, O_NONBLOCK);
+        return ret;
+}