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

fusl/src/unistd/dup2.c

Index: fusl/src/unistd/dup2.c
diff --git a/fusl/src/unistd/dup2.c b/fusl/src/unistd/dup2.c
new file mode 100644
index 0000000000000000000000000000000000000000..8f43c6ddfe8983e9565d6a14a10f423c9fd55f05
--- /dev/null
+++ b/fusl/src/unistd/dup2.c
@@ -0,0 +1,20 @@
+#include <unistd.h>
+#include <errno.h>
+#include <fcntl.h>
+#include "syscall.h"
+
+int dup2(int old, int new)
+{
+	int r;
+#ifdef SYS_dup2
+	while ((r=__syscall(SYS_dup2, old, new))==-EBUSY);
+#else
+	if (old==new) {
+		r = __syscall(SYS_fcntl, old, F_GETFD);
+		if (r >= 0) return old;
+	} else {
+		while ((r=__syscall(SYS_dup3, old, new, 0))==-EBUSY);
+	}
+#endif
+	return __syscall_ret(r);
+}