nixos-rebuild: Don't rely on $SHELL

Fixes #74.
stage-1: Clear environment before calling stage-2

In particular, stage-1's LD_LIBRARY_PATH should not infect stage-2.
Reported by viric.
Fix registering valid paths in VM tests

Commit 37b56574e20755e4c58370bb18a3d373d4ae7cfc revealed that the code
to get regInfo from /proc/cmdline was broken.  It only happened to
work because the kernel passes the command line to stage 1 through the
environment, so $regInfo was set anyway.
Revert 434260c772622408b1503a4ff2b65bfd900f3c80

There is a bug in unionfs, but it only manifested itself because of
our accidental use of stage-1 libraries in stage-2.
Set $HOME in stage-2

http://hydra.nixos.org/build/3900294
wpa_supplicant: Remove accidentally committed debug flag
Enable nix.readOnlyStore by default
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
Update the S3 AMI generation script
Wait a bit longer after EBS AMI registration