run_command and open_stream (napalm-device-types CommandChannelMixin,
which AccessPointDriver carries) run on an SSH exec channel next to
netmiko's shell: stdin, stderr apart, a real exit code. netOrk sent UCI
scripts and the wifi channel through the private _send_command, typing
a file in base64 chunks or printf lines; with stdin it writes it with
cat, and base64 is needed for nothing (netOrk #1113). OpenWrt has no
sudo, so a privileged command from a login other than root is refused
before it runs.
get_device_warnings goes with it: no_base64 was its last code.
