Post History

After addressing @Lundin 's suggestions:

- Implemented in terms of libc functions for performance.
- const correctness
- style improvements

I have one that I realized after trying to replace some existing code:

Much code out there has `char *end = buf + nitems(buf);` (although that should really be called `past_end`).  That's certainly easier to calculate than the real `end` that I used here, since you don't need to stick that `-1` all around, and therefore has less chance of an accident.

So, I tweaked `stpecpy()` to have the following definition:

```c
char *_Nonnull
stpecpy(char *_Nonnull dst,
        const char *_Nonnull restrict src,
        char *_Nonnull past_end)
{
	char *p;

	if (dst == past_end)
		return past_end;

	p = memccpy(dst, src, '\0', past_end - dst);
	if (p != NULL)
		return p - 1;

	/* truncation detected */
	past_end[-1] = '\0';
	return past_end;
}
```

The name of the function st**pe**cpy() seems to also have nice mnemonics for `past_end`.

Now the use is slightly simpler:

```c
int
main(void)
{
	ptrdiff_t  size = 10;
	char       buf[size];
	char      *past_end;
	ptrdiff_t  len;

	past_end = &buf[size];

	len = stpecpy(stpecpy(buf, "Hello", past_end), " world", past_end) - buf;
	if (len == size) {
		len--;
		puts("Following string is truncated.");
	}
	printf("%ti: %s\n", len, buf);

	len = stpecpy(stpecpy(stpecpy(buf, "Hello", past_end), " foo", end), "", past_end) - buf;
	if (len == size) {
		len--;
		puts("Following string is truncated.");
	}
	printf("%ti: %s\n", len, buf);

	len = stpecpy(stpecpy(stpecpy(buf, "Hello", past_end), " baar", end), "", past_end) - buf;
	if (len == size) {
		len--;
		puts("Following string is truncated.");
	}
	printf("%ti: %s\n", len, buf);

	len = stpecpy(stpecpy(buf, "H", past_end), "W", past_end) - buf;
	if (len == size) {
		len--;
		puts("Following string is truncated.");
	}
	printf("%ti: %s\n", len, buf);
}
```