9p: cope with bogus responses from server in p9_client_{read,write}
9p: cope with bogus responses from server in p9_client_{read,write}

if server claims to have written/read more than we'd told it to,
warn and cap the claimed byte count to avoid advancing more than
we are ready to.

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs