Be more careful about GucSource for internally-driven GUC settings.

The original advice for hard-wired SetConfigOption calls was to use
PGC_S_OVERRIDE, particularly for PGC_INTERNAL GUCs.  However,
that's really overkill for PGC_INTERNAL GUCs, since there is no
possibility that we need to override a user-provided setting.
Instead use PGC_S_DYNAMIC_DEFAULT in most places, so that the
value will appear with source = 'default' in pg_settings and thereby
not be shown by psql's new \dconfig command.  The one exception is
that when changing in_hot_standby in a hot-standby session, we still
use PGC_S_OVERRIDE, because people felt that seeing that in \dconfig
would be a good thing.

Similarly use PGC_S_DYNAMIC_DEFAULT for the auto-tune value of
wal_buffers (if possible, that is if wal_buffers wasn't explicitly
set to -1), and for the typical 2MB value of max_stack_depth.

In combination these changes remove four not-very-interesting
entries from the typical output of \dconfig, all of which people
fingered as "why is that showing up?" in the discussion thread.

Discussion: https://postgr.es/m/3118455.1649267333@sss.pgh.pa.us

src/backend/bootstrap/bootstrap.c

@@ -262,7 +262,7 @@ BootstrapModeMain(int argc, char *argv[], bool check_only)
 								(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
 								 errmsg("-X requires a power of two value between 1 MB and 1 GB")));
 					SetConfigOption("wal_segment_size", optarg, PGC_INTERNAL,
-									PGC_S_OVERRIDE);
+									PGC_S_DYNAMIC_DEFAULT);
 				}
 				break;
 			case 'c':