Replace memcpy() calls in xlog.c critical sections with struct assignments.

This gets rid of a dangerous-looking use of the not-volatile XLogCtl
pointer in a couple of spinlock-protected sections, where the normal
coding rule is that you should only access shared memory through a
pointer-to-volatile.  I think the risk is only hypothetical not actual,
since for there to be a bug the compiler would have to move the spinlock
acquire or release across the memcpy() call, which one sincerely hopes
it will not.  Still, it looks cleaner this way.

Per comment from Daniel Farina and subsequent discussion.