Fix alignment issues in FD passing code

Fix alignment issues in FD passing code

A char array on the stack is not guaranteed to have more than byte alignment.
This means that casting it to a 'struct cmsghdr' and accessing its members
may result in unaligned access.  This will generate SIGBUS on struct
alignment architectures like OpenBSD/sparc64.  The canonical solution is to
use a union to force proper alignment.

Signed-off-by: Mark Kettenis <kettenis@openbsd.org>
Reviewed-by: Matthieu Herrb <matthieu@herrb.eu>
Signed-off-by: Uli Schlachter <psychon@znc.in>