Refuse to start without IPC socket
Refuse to start without IPC socket

This change was (basically) suggested by @orestisfl in
https://github.com/i3/i3/pull/4638#issuecomment-958093518

Testing done:

$ I3SOCK=/dev/null DISPLAY=:2 LC_ALL=C build/i3
bind(): Address already in use
i3: Could not create the IPC socket: /dev/null

Signed-off-by: Uli Schlachter <346c0ad49ac8427b61b3b55abbe37dc0ebd524f2@znc.in>

i3/i3,Airblader/i3,i3/i3,stapelberg/i3,i3/i3,Airblader/i3,stapelberg/i3,stapelberg/i3,i3/i3,Airblader/i3,stapelberg/i3,Airblader/i3