install the docs with the makefile
fix hang on purge when all backups are removed
copy config files to DOCDIR in Makefile
Merge archive and backup configuration files

- Rename variables in archive_xlog and restore xlog to unify them
- Split pitr.conf in two sections
- Update restore_pitr to override the configuration with cli options
- Improve the creation of restore_command for restore_xlog in
  restore_pitr
- Fix error checking on backup search (harmless) in restore_pitr
- Add search for configuration file on its basename in restore_xlog
- Check the input files after logging setup in restore_xlog, to have the
  messages in syslog