launch systemd-logind if available
only launch logind in saucy and trusty
moar logind
installer: Make sure there is at least 1 target to be installed.

Ancient chroots did not provide a target list, this serves as a reminder.

Fixes #749 and #788.
Improve ze text.
test/16-targetsfile: Test if installer detects an absent or empty /etc/crouton/targets
use find to recursively umount /sys for bootstrap
Merge pull request #790 from drinkcat/update-error-no-target

installer: Make sure there is at least 1 target to be installed.
escape croutonversion call in enter-chroot
add basic logind test
don't rbind /sys if it's already bind-mounted

If enter-chroot is called twice for the same chroot, we don't want
slave mounts like the systemd cgroup to get blown away.
test: ensure systemd cgroup is set up in chroot
enter-chroot: mkdir /sys if needed
only run logind test if release is supported
Merge pull request #699 from smibarber/logind

Launch systemd-logind if available
mkdir /sys at a reasonable time
Merge pull request #816 from smibarber/howdoimkdirsys

mkdir /sys at a reasonable time
croutonversion: Print error message on wget failure
Merge pull request #820 from drinkcat/croutonversion-printerror

croutonversion: Print error message on wget failure
use ~/Downloads as home directory for chroot user