Config reload/restart preview

Hi,

I've noticed that there's no easy way of checking which settings will
change if the config is reloaded, and I think not being able to do this can
cause some unfortunate problems.

For example, a customer went to change their configuration, just setting
log_autovacuum_min_duration to about 20 seconds, and reloaded the server.
However, the log file swelled to over 5GB in size before they realised
something was wrong, and then reverted the change. It transpired that the
reload also pulled in a log_statements change from 'ddl' to 'all' that
someone must have changed at some point without applying it.

Should we have a way of previewing changes that would be applied if we
reloaded/restarted the server?

For example:

pg_ctl previewconfig

SIGHUP: log_statements will change from 'ddl' to 'all'
SIGHUP: log_vacuum_min_duration will change from -1 to 20000
POSTMASTER: fsync will change from 'on' to 'off'

I'm not proposing this specifically, but something that would provide such
information.

--
Thom