fix: run privileged commands on the exec channel, never typed into the shell

_sudo typed `echo '<password>' | sudo -S -p "" <command>` into netmiko's
interactive shell. bash kept it in its history, so every poll wrote the sudo
password into the login user's ~/.bash_history, and while the command ran it
was in the process list for every local user (NetOrk/netork#1039).

_sudo, _sudo_status and _run_privileged with a password now run on an exec
channel like run_command(privileged=True): the password reaches sudo -S on
stdin, there is no PTY to echo it, and the script starts with
`exec </dev/null` so a non-prompting sudo leaves it unread. The whole
command runs as root in one sh -c - before, only the part up to the first
`;`, `&&` or `|` did. The exit status comes from the channel.

The interactive session switches its history off before its first command.
get_vpn_tunnels with a password reads `wg show all dump` from the exec
channel instead of a temp file that held the private keys.

README.md

@@ -141,6 +141,11 @@ The SSH user needs read access to:
 For full functionality it is recommended to run as `root` or grant passwordless `sudo` for
 the above commands.
 
+With a `sudo_password`, privileged commands run on an exec channel next to the interactive
+shell: the password reaches `sudo -S` on stdin, never a command line, so it shows neither in
+the process list nor in a shell history. The interactive session switches its history off
+before its first command.
+
 `get_services()` and `manage_service()` come from napalm-device-types'
 `SystemdServicesMixin`; this driver supplies only the transport. An action runs as
 `timeout 45 systemctl --no-ask-password <action> -- <unit>.service`, so a unit that hangs