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

fusl/src/string/stpncpy.c

+#include <string.h>
+#include <stdint.h>
+#include <limits.h>
+#include "libc.h"
+
+#define ALIGN (sizeof(size_t)-1)
+#define ONES ((size_t)-1/UCHAR_MAX)
+#define HIGHS (ONES * (UCHAR_MAX/2+1))
+#define HASZERO(x) ((x)-ONES & ~(x) & HIGHS)
+
+char *__stpncpy(char *restrict d, const char *restrict s, size_t n)
+{
+        size_t *wd;
+        const size_t *ws;
+
+        if (((uintptr_t)s & ALIGN) == ((uintptr_t)d & ALIGN)) {
+                for (; ((uintptr_t)s & ALIGN) && n && (*d=*s); n--, s++, d++);
+                if (!n || !*s) goto tail;
+                wd=(void *)d; ws=(const void *)s;
+                for (; n>=sizeof(size_t) && !HASZERO(*ws);
+                       n-=sizeof(size_t), ws++, wd++) *wd = *ws;
+                d=(void *)wd; s=(const void *)ws;
+        }
+        for (; n && (*d=*s); n--, s++, d++);
+tail:
+        memset(d, 0, n);
+        return d;
+}
+
+weak_alias(__stpncpy, stpncpy);
+