Revert "Enable nix.readOnlyStore by default"

This reverts commit 683100666d98f758b4ae3172686633a4840ec1bc.

Seems somebody (systemd? the kernel?) gets confused at power
events and remounts the filesystem containing /nix/store as
read-only.
xserver.nix: Check whether Polkit is enabled

Our X session script requires Polkit because it calls systemd-inhibit.
Turn on nix.readOnlyStore again
nix-daemon: Put back the --daemon flag

That flag doesn't do anything, except that it allows setting the
client PID in the process title of the children.
Fix description of powerManagement.cpuFreqGovernor
Start a getty on tty1 again
Use systemd.targets to define ip-up
Get the VirtualBox image generation to work again
Update the EBS creation script
Put the root FS of S3-backed instances on ext4
spamassassin: convert module to systemd