try to detect sudo spawned root-shell in prefixing

try to detect sudo spawned root-shell in prefixing

It is a try as the we need to inspect SUDO_COMMAND which could be
anything – apt, apt-get, in /usr/bin, in a $DPKG_ROOT "chroot", build
from source, aliases, …

The best we can do is look if the SHELL variable is equal to the
SUDO_COMMAND which would mean a shell was invoked. That isn't fail-safe
if different shells are involved as sub-shells have the tendency of not
overriding the SHELL so a bash started from within zsh can happily
pretend to be still zsh, so we could have a look at /etc/shells for a
list, but oh well, we have to stop somewhere I guess.
This sudo-prefixing feature is a gimmick after all.

Closes: 825742