dnsheader_aligned: Prevent copies

dnsheader_aligned: Prevent copies

The `dnsheader_aligned` object contains a pointer that references
either the `dnsheader` passed to the constructor if it is properly
aligned, or the internal `dnsheader` member. In the second case,
making a copy would mean we can reference an object that has been
destructed, which is a serious problem.

This commit also ensures copy elision is done `DNSQuestion:getHeader`,
as otherwise the compiler might refuse to compile.

Signed-off-by: Remi Gacogne <remi.gacogne@powerdns.com>