sys/socket.h: fix some CMSG_ macros not aligning

[ArsenArsen]

Excerpt from sys/socket.h:
	// Control message format:
	// The offsets marked with ^ are aligned to alignof(size_t).
	//
	// |---HEADER---|---DATA---|---PADDING---|---HEADER---|...
	// ^            ^                        ^
	// |---------CMSG_LEN------|
	// |---------------CMSG_SPACE------------|

By this definition, control message header and data should both be
aligned to (at least) alignof(size_t), however, previously, the
implementation of these macros in options/posix relied on
sizeof(struct cmsghdr) % sizeof(size_t) being zero. After the recent
change to socklen_t, this no longer held true, and DATA would be aligned
to a 12 byte boundary (on x86_64), and hence would move all data back a
bit. Code that relied on CMSG_ALIGN() to compute sizes of cmsg data
(notably, sysdeps/managarm) hence broke as a result.

The change in this commit fixes that misalignment.

It would be desirable for this change to land into 3.0.0-rc2