Overhaul default hosts configuration.

• When no virtualhosts are defined, and the default localhost/localhost is effective, the post-install script now adds DEFAULT_EMAIL_HOST, DEFAULT_URL_HOST, and add_virtualhost() lines to mm_cfg.py.
  
  Add corresponding support to remove unchanged configurations to the deinstall script.

• While here, overhaul crontab configuration to only request a merge if the crontab had been modified, and unify progress reports.

• Merge pkg-deinstall into pkg-install, to avoid duplication of code that might be missed in later maintenance.

PR: 225961
Reported by: Terry Kennedy