Make file descriptor passing work for CloudABI's sendmsg().

Reduce the potential amount of code duplication between cloudabi32 and
cloudabi64 by creating a cloudabi_sock_recv() utility function. The
cloudabi32 and cloudabi64 modules will then only contain code to convert
the iovecs to the native pointer size.

In cloudabi_sock_recv(), we can now construct an SCM_RIGHTS cmsghdr in
an mbuf and pass that on to kern_sendit().