Ensure that empty NameLists always return
a zero length []string, not nil.

In practice NameLists are only used in a few
message types and always consumed by a for
range function so the difference between nil
and []string{} is not significant.

Also, add exp/ssh to pkg/Makefile as suggested
by rsc.

R=rsc, agl
CC=golang-dev
https://golang.org/cl/5400042