Undo accidental commit
modules/services/networking/ssh/sshd.nix: configure AddressFamily properly

Explicitly restrict sshd to use of IPv4 addresses if IPv6 support is not enabled.
modules/programs/ssh.nix: configure AddressFamily properly

Explicitly restrict ssh clients to use of IPv4 addresses if IPv6 support is not enabled.
modules/programs/ssh.nix: simplify expression that generates 'ForwardX11' entry
modules/programs/ssh.nix: strip trailing whitespace
Remove some obsolete options
In the tests, don't start agetty on /dev/ttyS0

Running agetty on ttyS0 interferes with the backdoor, which uses ttyS0
as its standard error.  After agetty starts, writes to the stderr file
descriptor will return EIO (though doing "exec 2>/proc/self/fd/2" will
miracuously fix this).

http://hydra.nixos.org/build/3252782
Added environment.promptInit to allow PS1 overriding. Would be nicer to be able to allow overriding via shellInit, however could not get that to work. For now this is a temporary solution which will not break anything.
In headless deployments, don't start agetty on the console
Logstash: do not always log to stdout
Logstash: pass TZ, redirect log output to prevent recursion when using syslogd.
Logstash: fix typo, should have tested.
Fix the installer test

http://hydra.nixos.org/build/3253038
Kill the backdoor more forcefully

Otherwise it hangs until the 90 second timeout expires.

http://hydra.nixos.org/build/3253068
Test driver: Don't wait for a reply after issuing "poweroff"
Fix coverage data collection

http://hydra.nixos.org/build/3253046
dhcpcd: Don't duplicate log messages

Dhcpcd writes log messages to both syslog and stderr.  So ignore
stderr.
dhcpcd: Go into the background immediately
Order mkfs services before the corresponding fsck services
Make Apache wait for ‘charon send-keys’

(This is a no-op on non-Charon deployments since the ‘keys.target’
unit won't have any dependencies.)