Improving display of octal GUCs

It's customary to write the values of unix_socket_permissions
and a few other GUCs in octal. However, we implemented that
via show_hooks, which don't affect any pg_settings columns
except the current value. So you get results like this:

regression=# SELECT name,setting,min_val,max_val,boot_val,reset_val
FROM pg_settings
WHERE name IN ('data_directory_mode', 'log_file_mode', 'unix_socket_permissions');
name | setting | min_val | max_val | boot_val | reset_val
-------------------------+---------+---------+---------+----------+-----------
data_directory_mode | 0700 | 0 | 511 | 448 | 448
log_file_mode | 0600 | 0 | 511 | 384 | 384
unix_socket_permissions | 0777 | 0 | 511 | 511 | 511
(3 rows)

That's always been a niggling annoyance, and we recently got a bug
report complaining about it [1]. So here's an attempt to improve
matters:

regression=# SELECT name,setting,min_val,max_val,boot_val,reset_val
FROM pg_settings
WHERE name IN ('data_directory_mode', 'log_file_mode', 'unix_socket_permissions');
name | setting | min_val | max_val | boot_val | reset_val
-------------------------+---------+---------+---------+----------+-----------
data_directory_mode | 0700 | 0000 | 0777 | 0700 | 0700
log_file_mode | 0600 | 0000 | 0777 | 0600 | 0600
unix_socket_permissions | 0777 | 0000 | 0777 | 0777 | 0777
(3 rows)

Details in the draft commit message.

regards, tom lane

[1] https://www.postgresql.org/message-id/flat/19540-e641040b089ab768%40postgresql.org