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

fusl/src/unistd/dup3.c

Index: fusl/src/unistd/dup3.c
diff --git a/fusl/src/unistd/dup3.c b/fusl/src/unistd/dup3.c
new file mode 100644
index 0000000000000000000000000000000000000000..0eb6caf59e6e1b0015b231632b6c20f879c5ad95
--- /dev/null
+++ b/fusl/src/unistd/dup3.c
@@ -0,0 +1,25 @@
+#define _GNU_SOURCE
+#include <unistd.h>
+#include <errno.h>
+#include <fcntl.h>
+#include "syscall.h"
+#include "libc.h"
+
+int __dup3(int old, int new, int flags)
+{
+	int r;
+#ifdef SYS_dup2
+	if (old==new) return __syscall_ret(-EINVAL);
+	if (flags & O_CLOEXEC) {
+		while ((r=__syscall(SYS_dup3, old, new, flags))==-EBUSY);
+		if (r!=-ENOSYS) return __syscall_ret(r);
+	}
+	while ((r=__syscall(SYS_dup2, old, new))==-EBUSY);
+	if (flags & O_CLOEXEC) __syscall(SYS_fcntl, new, F_SETFD, FD_CLOEXEC);
+#else
+	while ((r=__syscall(SYS_dup3, old, new, flags))==-EBUSY);
+#endif
+	return __syscall_ret(r);
+}
+
+weak_alias(__dup3, dup3);