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

fusl/src/unistd/ttyname_r.c

Index: fusl/src/unistd/ttyname_r.c
diff --git a/fusl/src/unistd/ttyname_r.c b/fusl/src/unistd/ttyname_r.c
new file mode 100644
index 0000000000000000000000000000000000000000..8bac7b2f3ab10cb0929341ca1425af65c8d409bf
--- /dev/null
+++ b/fusl/src/unistd/ttyname_r.c
@@ -0,0 +1,22 @@
+#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;
+	}
+}