ssh: xstrdup(): use memcpy(3)

ssh: xstrdup(): use memcpy(3)

Copying the given string into the buffer with strlcpy(3) confers no
benefit in this context because we have already determined the
string's length with strlen(3) in order to allocate that buffer.

Thread: https://marc.info/?l=openbsd-tech&m=164687525802691&w=2

ok dtucker@ millert@