Avoid member access within misaligned address for ancillary data buffer.

For glibc-2.23, the CMSG_FIRSTHDR macro is defined as

	#define CMSG_FIRSTHDR(mhdr) \
	  ((size_t) (mhdr)->msg_controllen >= sizeof (struct cmsghdr)                 \
	   ? (struct cmsghdr *) (mhdr)->msg_control : (struct cmsghdr *) 0)

In recv_cred_buffer(), pass_afd() and dispose_fds() the on-stack
ancillary data buffer is not necessarily aligned. The pointer is
cast to struct cmsghdr *, then dereferenced, resulting in undefined
behaviour due to the lack of alignment.

This patch asks the compiler to align the ancillary data buffers.