run_command(privileged=True, stdin=data) sent `sudo -S` with stdin =
<sudo password>\n + data. A sudo that does not prompt (NOPASSWD, a cached
timestamp) leaves the password line on stdin, and the command reads it as
the first line of the data - `cat > file` wrote it into the file
(NetOrk/netork#1029).
run_command now raises ValueError for that combination before opening a
channel. Stdin still reaches the command under `sudo -n` (no password set),
for a root login, and for unprivileged commands. Callers stage the data
without privilege and have root install the file. open_stream documents the
same caveat.
Christian Manivong