Don't rely on getlogin() for getting usernames.

In some cases this will, for whatever reason, return NULL. As a result the fsapi
path ends up being NULL terminated. This results in any following data being
ignored by whatever uses the path. This caused the console to use paths such as
"/tmp/rbx-/console-response" instead of
"/tmp/rbx-yorickpeterse-XXX/console-response".