setup mode should have an no-act option only displaying the required changes

[mbanck]

Running pg_backup_ctl [...] setup modifies the postgresql.conf file. However, at least the default as suggested by the README does not mention which option were changed specifically. Furthermore, maybe the user keeps postgresql.conf in configuration management and prefers to make the required changes on their own terms.

So I suggest an additional option to setup which only mentions which options would be changed or added, but does not actually do the change.