Per POSIX, <sys/socket.h> needs to expose struct iovec, and may do so

Per POSIX, <sys/socket.h> needs to expose struct iovec, and may do so
by pulling in <sys/uio.h>, so do so.  Remove some stuff that we can trust
<sys/uio.h> to also provide, like cdefs.h, _types.h, and exposing size_t
and ssize_t

pointed out by naddy@
ok deraadt@