[krbdev.mit.edu #6010] krb5int_gic_opte_copy should copy elements individually

[Tom Yu via RT]

On most platforms, the memcpy() should work because the structure
layouts are identical for their initial overlapping elements.  (The C
standard does not guarantee this.)  On the Mac, the use of "#pragma
pack" could cause a difference in structure layout.

We need to investigate whether this constitutes an ABI change on the
Mac.  Given that a caller can receive a new structure type-punned to
the old one, I believe it is an ABI change.