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

fusl/src/multibyte/mbsnrtowcs.c

Index: fusl/src/multibyte/mbsnrtowcs.c
diff --git a/fusl/src/multibyte/mbsnrtowcs.c b/fusl/src/multibyte/mbsnrtowcs.c
new file mode 100644
index 0000000000000000000000000000000000000000..68b9960f375e3b3dcbe5f4f5de9d6a8a543be104
--- /dev/null
+++ b/fusl/src/multibyte/mbsnrtowcs.c
@@ -0,0 +1,59 @@
+/* 
+ * 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 mbsnrtowcs(wchar_t *restrict wcs, const char **restrict src, size_t n, size_t wn, mbstate_t *restrict st)
+{
+	size_t l, cnt=0, n2;
+	wchar_t *ws, wbuf[256];
+	const char *s = *src;
+
+	if (!wcs) ws = wbuf, wn = sizeof wbuf / sizeof *wbuf;
+	else ws = wcs;
+
+	/* making sure output buffer size is at most n/4 will ensure
+	 * that mbsrtowcs never reads more than n input bytes. thus
+	 * we can use mbsrtowcs as long as it's practical.. */
+
+	while ( s && wn && ( (n2=n/4)>=wn || n2>32 ) ) {
+		if (n2>=wn) n2=wn;
+		n -= n2;
+		l = mbsrtowcs(ws, &s, n2, st);
+		if (!(l+1)) {
+			cnt = l;
+			wn = 0;
+			break;
+		}
+		if (ws != wbuf) {
+			ws += l;
+			wn -= l;
+		}
+		cnt += l;
+	}
+	if (s) while (wn && n) {
+		l = mbrtowc(ws, s, n, st);
+		if (l+2<=2) {
+			if (!(l+1)) {
+				cnt = l;
+				break;
+			}
+			if (!l) {
+				s = 0;
+				break;
+			}
+			/* have to roll back partial character */
+			*(unsigned *)st = 0;
+			break;
+		}
+		s += l; n -= l;
+		/* safe - this loop runs fewer than sizeof(wbuf)/8 times */
+		ws++; wn--;
+		cnt++;
+	}
+	if (wcs) *src = s;
+	return cnt;
+}