Further tweak the default behavior of psql's \dconfig.

Further tweak the default behavior of psql's \dconfig.

Define "parameters with non-default settings" as being those that
not only have pg_settings.source different from 'default', but
also have a current value different from the hard-wired boot_val.
Adding the latter restriction removes a number of not-very-interesting
cases where the active setting is chosen by initdb but in practice
tends to be the same all the time.

Per discussion with Jonathan Katz.

Discussion: https://postgr.es/m/YlFQLzlPi4QD0wSi@msg.df7cb.de