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

fusl/src/unistd/ttyname_r.c

+#include <unistd.h>
+#include <errno.h>
+
+void __procfdname(char *, unsigned);
+
+int ttyname_r(int fd, char *name, size_t size)
+{
+        char procname[sizeof "/proc/self/fd/" + 3*sizeof(int) + 2];
+        ssize_t l;
+
+        if (!isatty(fd)) return ENOTTY;
+
+        __procfdname(procname, fd);
+        l = readlink(procname, name, size);
+
+        if (l < 0) return errno;
+        else if (l == size) return ERANGE;
+        else {
+                name[l] = 0;
+                return 0;
+        }
+}