bug#36583: 26.1; emacs 26 tries loading original user's .emacs after su to root

[Steinar Bang]

Another breakage discovered today: it's impossible to do X forwarding
with an unbroken emacs configuration.

If you do:
 - ssh -Y someserver
 - /bin/su - root
then the DISPLAY variable set by "ssh -Y" is lost and it's impossible to
do X forwarding.

Doing just "/bin/su" (which I used to do before the last debian upgrade)
gives me a broken emacs configuration (because ~sb/.emacs is used but
ELPA packages under ~sb/.emacs.d/ are not used).

(And as Lars has pointed out elsewhere in this thread: the emacs
configuration of root should be tighter and more security conscious than
the configuration of a regular user. So just including the .emacs.d/ of
the original user may not be the appropriate fix)