installer: Allow host-bin restoration only.

Add a  `-b` option to the installer that only restores host-bin scripts, based on the target files installed in all existing chroots. This happens to be a prerequisite for #706 .

Incidentally, this fixes #1106 as crouton -f tarball now only restores host-bin (and not chroot-bin, which would mangle croutonversion output, see https://github.com/dnschneid/crouton/issues/1106#issuecomment-59576779).

We also provide a minimal test for `-b`: binaries are restored.

I've read somewhere that you need to have a failing test before fixing issues. This confirms #1106:
`2014-10-20_03-49-08_drinkcat_chroagh_splittest+edit-chroot-fix.testonly_12` (only e62f4b1)

Then a full test:
`2014-10-20_06-01-54_drinkcat_chroagh_splittest+edit-chroot-fix`
another one that tests all targets (`x0`):
`2014-10-20_06-02-35_drinkcat_chroagh_splittest+edit-chroot-fix_x0` (mirrors are flaky, but overall it's ok)
and a last one with the latest cosmetic changes:
`2014-10-20_17-13-16_drinkcat_chroagh_splittest+edit-chroot-fix_12`
`.crouton-targets` can be empty after a installer fail (very early Ctrl-C, script bug, or perhaps more importantly if an unsupported target is specified).

Less importantly:
 - `/etc/crouton/targets` only contains explicitly specified targets (no deps): shouldn't be an issue in most cases.
 - `.crouton-targets` gets broken on `-U` updates (again, shouldn't be an issue)