char *copy(char *dest, const char *src, int capacity) {

    size_t i;

    for (i = 0; i < capacity && src[i] != '\0'; i++){
        dest[i] = src[i];
        for ( ; i < capacity; i++){
            dest[i] = '\0';
        }
    }
        

    return dest;
}
Write the copy function to perform exactly as strncpy does, with one
   exception: your copy function will guarantee that dest is always
   null-terminated. Capacity is expected to be the number of bytes of 
   memory allocated to dest.
   You shoud read the man page to learn how strncpy works.

  NOTE: You must write this function without using any string functions.
  The only function that should depend on string.h is memset.