bug#22930: X11 forwarding fails without /tmp/.X11-unix directory

[myglc2]

Example: using guix commit 59a4dd5 with openssh xauth & emacs in the
user profile (c02glc.scm) and a minimal system profile (c02system.scm),
after a reboot we see ...

nemo:~ $ ssh g1
address@hidden's password:
X11 forwarding request failed on channel 0

... and in /var/log/messages ...

Mar  6 19:23:35 localhost lshd[372]: lshd: Failed to cd to `/tmp/.X11-unix' 
(errno = 2): No such file or directory 
Mar  6 19:23:35 localhost lshd[372]: lshd: Updating utmp for login failed 
(errno = 2): No such file or directory 

If we do ...

mkdir /tmp/.X11-unix

... logout and login, forwarding works, emacs runs in X, and
/var/log/messages is clean. If we reboot, X11 forwarding again fails.

c02glc.scm
Description: Binary data

c02system.scm
Description: Binary data