* debian/apt.conf.autoremove: don't include linux-image*,

* debian/apt.conf.autoremove: don't include linux-image*,
  linux-restricted-modules*, and linux-ubuntu-modules* packages in the
  list to never be autoremoved.
* debian/apt.auto-removal.sh, debian/rules, debian/apt.dirs: install new
  script to /etc/kernel/postinst.d/ which ensures we only automatically
  keep the currently-running kernel, the being-installed kernel, and the
  newest kernel, so we don't fill /boot up with an unlimited number of
  kernels.  LP: #923876.