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/include/utils/guc.h

@@ -83,8 +83,7 @@ typedef enum
  * override the postmaster command line.)  Tracking the source allows us
  * to process sources in any convenient order without affecting results.
  * Sources <= PGC_S_OVERRIDE will set the default used by RESET, as well
- * as the current value.  Note that source == PGC_S_OVERRIDE should be
- * used when setting a PGC_INTERNAL option.
+ * as the current value.
  *
  * PGC_S_INTERACTIVE isn't actually a source value, but is the
  * dividing line between "interactive" and "non-interactive" sources for
@@ -99,6 +98,11 @@ typedef enum
  * shouldn't throw hard errors in this case, at most NOTICEs, since the
  * objects might exist by the time the setting is used for real.
  *
+ * When setting the value of a non-compile-time-constant PGC_INTERNAL option,
+ * source == PGC_S_DYNAMIC_DEFAULT should typically be used so that the value
+ * will show as "default" in pg_settings.  If there is a specific reason not
+ * to want that, use source == PGC_S_OVERRIDE.
+ *
  * NB: see GucSource_Names in guc.c if you change this.
  */
 typedef enum