lib9: avoid all -Wconversion warnings

lib9: avoid all -Wconversion warnings

Built after adding -Wconversion to the list of compiler
arguments used when building.  I believe these are all OK
assuming we will not change the API.  There is no effort to
detect overflow due to very long strings.

R=golang-dev, dave, rsc, r
CC=golang-dev
https://golang.org/cl/10195044