More doc updates
ensure that the default pre-backup hook is completely silent
  The pre-backup hook is injected into the rdiff-backup
  remote-schema and its output will be interpreted as
  rdiff-backup output.
hardcode lib and conf file paths and add set -eu
cleanup the main execution handler
implement `rdbmanage config': print current config
remove all usage of unbound variables: rely on $#
  `[[-z $unbound_var ]]' breaks with `set -o nounset'
improve loggin and use `printf' instead of `echo'
cleanup rdbmanage.conf and fix typos
Use a lighter default in rdb_default_filelist
cleanup rdbmanage_lib.sh
  - replace `echo' usage with `printf'
  - improve logging
  - use consistent tmp files for cron manipulation
do not use awk's systime() function
  systime() is only defined in GNU-Awk, and not, for example, in mawk.
update README.md
Add an install script
Cleanup installation/path configuration
  - install rdbmanage into /usr/share/rdbmanage
  - symlink /usr/bin/rdbmanage -> ../share/rdbmanage/rdbmanage
  - remove configuration variables containing installation file paths
  - update README.md
Add a Dockerfile
remove stale files and fix minor typos
cleanup logging and admin notification logic
ensure that the crontab executable has the proper g+s bit