memcpy is not equivalent to memmove (which is used by vim_strcat), this
could cause subtle bugs if xstrlcat is used as a replacement for
vim_strcat. But vim_strcat is inconsistent: in the `else` branch it uses
strcpy, which doesn't allow overlap.
Helped-by: oni-link <knil.ino@gmail.com>
Helped-by: James McCoy <jamessan@jamessan.com>
Helped-by: Nikolai Aleksandrovich Pavlov <kp-pav@yandex.ru>
