Support GCC's transparent unions

Support GCC's transparent unions

This stops warnings in code using socket operations with a modern glibc,
which otherwise result in warnings of the form:

	warning: incorrect type in argument 2 (invalid types)
	   expected union __CONST_SOCKADDR_ARG [usertype] __addr
	   got struct sockaddr *<noident>

Since transparent unions are only applicable to function arguments, we
create a new function to check that the types are compatible
specifically in this context.

Also change the wording of the existing warning slightly since sparse
does now support them.  The warning is left in case people want to avoid
using transparent unions.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Christopher Li <sparse@chrisli.org>