I wasn't properly checking for rollover in iovec_copyin(). Remove code

I wasn't properly checking for rollover in iovec_copyin().  Remove code
that ineffectively checks for rollover by checking if a size_t was ever
less than zero (duh).

Add the proper error checking to kern_sendmsg() and kern_recvmsg() which
recieve a uio structure that was populated with the aid of iovec_copyin().