add fallback emulation for accept4 on old kernels

the other atomic FD_CLOEXEC interfaces (dup3, pipe2, socket) already
had such emulation in place. the justification for doing the emulation
here is the same as for the other functions: it allows applications to
simply use accept4 rather than having to have their own fallback code
for ENOSYS/EINVAL (which one you get is arch-specific!) and there is
no reasonable way an application could benefit from knowing the
operation is emulated/non-atomic since there is no workaround at the
application level for non-atomicity (that is the whole reason these
interfaces were added).

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)
 {
-	return socketcall_cp(accept4, fd, addr, len, flg, 0, 0);
+	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;
 }