cork_strndup should copy entire string

The `cork_strndup` and `cork_xstrndup` functions were using `strncpy`
under the covers to make the copy of the incoming string.  This
truncates the copy at the first NUL byte.  We want to copy the whole
thing, so we should use `memcpy` instead.
Merge branch 'feature/strndup' into develop

* feature/strndup:
  cork_strndup should copy entire string