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

src/bin/psql/describe.c

@@ -4408,7 +4408,8 @@ describeConfigurationParameters(const char *pattern, bool verbose,
 							  NULL, "pg_catalog.lower(s.name)", NULL,
 							  NULL);
 	else
-		appendPQExpBufferStr(&buf, "WHERE s.source <> 'default'\n");
+		appendPQExpBufferStr(&buf, "WHERE s.source <> 'default' AND\n"
+							 "      s.setting IS DISTINCT FROM s.boot_val\n");
 
 	appendPQExpBufferStr(&buf, "ORDER BY 1;");