Fix ‘nixos-container run’

By default, socat only waits 0.5s for the remote side to finish after
getting EOF on the local side. So don't close the local side, instead
wait for socat to exit when the remote side finishes.

http://hydra.nixos.org/build/10663282
(cherry picked from commit fec3b75e4bba045178bfb4b9ba1e2aa8c55447f5)
Make some tests release-critical

(cherry picked from commit 537c034e8fd4568de43c38de3becb985e7d33e63)
Give the KDE test more memory

(cherry picked from commit 7ddcd7b6b69c45739776db5f797a3956c8f5dd78)
switch-to-configuration: Use systemctl's --no-legend flag

(cherry picked from commit 37e6e08cdee082d9f60ab2900c960f2f594933a0)
switch-to-configuration: Use old systemctl to stop units

Otherwise, when switching from systemd 203 to 212, you get errors like:

  Failed to stop remote-fs.target: Bad message
  Failed to stop systemd-udevd-control.socket: Bad message
  ...

(cherry picked from commit 56b4b841ae45c9fb512096852d06410e7ebe45d2)