add implementation of memccpy function

src/string/memccpy.c

+#include <string.h>
+#include <stdlib.h>
+#include <stdint.h>
+#include <limits.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)
+
+void *memccpy(void *dest, const void *src, int c, size_t n)
+{
+	unsigned char *d = dest;
+	const unsigned char *s = src;
+	size_t *wd, k;
+	const size_t *ws;
+
+	c = (unsigned char)c;
+	if (((uintptr_t)s & ALIGN) == ((uintptr_t)d & ALIGN)) {
+		for (; ((uintptr_t)s & ALIGN) && n && (*d=*s)!=c; n--, s++, d++);
+		if ((uintptr_t)s & ALIGN) goto tail;
+		k = ONES * c;
+		wd=(void *)d; ws=(const void *)s;
+		for (; n>=sizeof(size_t) && !HASZERO(*ws^k);
+		       n-=sizeof(size_t), ws++, *wd++) *wd = *ws;
+		d=(void *)wd; s=(const void *)ws;
+	}
+	for (; n && (*d=*s)!=c; n--, s++, d++);
+tail:
+	if (*s==c) return d+1;
+	return 0;
+}