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

fusl/src/multibyte/wcsnrtombs.c

Index: fusl/src/multibyte/wcsnrtombs.c
diff --git a/fusl/src/multibyte/wcsnrtombs.c b/fusl/src/multibyte/wcsnrtombs.c
new file mode 100644
index 0000000000000000000000000000000000000000..ee4a534ab0c5131991a9af15c612f63391cf8d27
--- /dev/null
+++ b/fusl/src/multibyte/wcsnrtombs.c
@@ -0,0 +1,47 @@
+/* 
+ * This code was written by Rich Felker in 2010; no copyright is claimed.
+ * This code is in the public domain. Attribution is appreciated but
+ * unnecessary.
+ */
+
+#include <wchar.h>
+
+size_t wcsnrtombs(char *restrict dst, const wchar_t **restrict wcs, size_t wn, size_t n, mbstate_t *restrict st)
+{
+	size_t l, cnt=0, n2;
+	char *s, buf[256];
+	const wchar_t *ws = *wcs;
+
+	if (!dst) s = buf, n = sizeof buf;
+	else s = dst;
+
+	while ( ws && n && ( (n2=wn)>=n || n2>32 ) ) {
+		if (n2>=n) n2=n;
+		wn -= n2;
+		l = wcsrtombs(s, &ws, n2, 0);
+		if (!(l+1)) {
+			cnt = l;
+			n = 0;
+			break;
+		}
+		if (s != buf) {
+			s += l;
+			n -= l;
+		}
+		cnt += l;
+	}
+	if (ws) while (n && wn) {
+		l = wcrtomb(s, *ws, 0);
+		if ((l+1)<=1) {
+			if (!l) ws = 0;
+			else cnt = l;
+			break;
+		}
+		ws++; wn--;
+		/* safe - this loop runs fewer than sizeof(buf) times */
+		s+=l; n-=l;
+		cnt += l;
+	}
+	if (dst) *wcs = ws;
+	return cnt;
+}