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

fusl/src/unistd/dup2.c

+#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);
+}