These are the components that the installer manages, and the contexts in which
the installation/upgrade/migration must work.

- ownership+permissions
  - reporting directories - get from config or /var/www/html/weewx
    - ownership on reporting directories
    - permissions on reporting directories
  - database directory - /var/lib/weewx
    - ownership on database directory and all .sdb files
    - permissions on database directory and all .sdb files
  - config directory - /etc/weewx
    - ownership on config directory
    - permissions on config directory

- new install
  - install defaults
  - ownership+permissions
  - install systemd unit
  - install systemd template
  - enable weewxd; start weewxd

- upgrade from 5.x
  - do not touch defaults
  - ownership+permissions
  - systemd unit
  - systemd template instances
  - if running, then stop/start, else nothing

- upgrade from 4.x
  - update defaults
  - ownership+permissions
  - convert init.d/weewx to systemd unit
  - convert init.d/weewx-multi to systemd template instances
  - if running, then stop/start, else nothing