libutil: Remove free() from strnsub()

The free() was added in the wrong assumption that xargs was not
freeing the argument in the position replaced, but it actually
does it.

Also, removing the call to free() makes more general the function.

libutil/strnsubst.c

@@ -56,7 +56,6 @@ strnsubst(char **str, const char *match, const char *replstr, size_t maxsize)
 	}
 	strlcpy(s2 + s2len, s1, maxsize - s2len);
 done:
-	free(*str);
 	*str = s2;
 	return;
 }