Re: display hot standby state in psql prompt

Hi Jim,

+1 for the patch,i have reviewed and tested this patch, except these below cosmetic changes it LGTM.

cosmetic changes:
1) add comment about %i in get_prompt api.
2) maybe we can use read-write instead of read/write to be consistent with the
    naming such as options of target_session_attrs uses read-write.

testing:

=> in primary node:

psql (19devel)
Type "help" for help.

postgres=# \set PROMPT1 '[%i] # '
[read/write] # set default_transaction_read_only=on;
SET
[read-only] # set default_transaction_read_only=off;
SET
[read/write] # show in_hot_standby ;
 in_hot_standby
----------------
 off
(1 row)

[read/write] # set default_transaction_read_only=on;
SET
[read-only] # show in_hot_standby ;
 in_hot_standby
----------------
 off
(1 row)

[read-only] # \q

=> in replica node

psql (19devel)
Type "help" for help.

postgres=# \set PROMPT1 '[%i] # '
[read-only] # show in_hot_standby ;
 in_hot_standby
----------------
 on
(1 row)

[read-only] # show default_transaction_read_only;
 default_transaction_read_only
-------------------------------
 off
(1 row)

[read-only] # set default_transaction_read_only=on;
SET
[read-only] # set transaction_read_only=on;
SET
[read-only] # set transaction_read_only=off;
ERROR:  cannot set transaction read-write mode during recovery
[read-only] # select pg_promote();
 pg_promote
------------
 t
(1 row)

[read-only] # show in_hot_standby ;
 in_hot_standby
----------------
 off
(1 row)

[read-only] # show default_transaction_read_only;
 default_transaction_read_only
-------------------------------
 on
(1 row)

[read-only] # set default_transaction_read_only=off;
SET
[read/write] #