pgsql: Add %x to default PROMPT1 and PROMPT2 in psql

Add %x to default PROMPT1 and PROMPT2 in psql

%d can be used to track if the current connection is in a transaction
block or not, and adding it by default to the prompt has the advantage
to not need a modification of .psqlrc, something not possible depending
on the environment.

This discussion has happened across various sources, and there was a
strong consensus in favor of this change.

Author: Vik Fearing
Reviewed-by: Fabien Coelho
Discussion: https://postgr.es/m/09502c40-cfe1-bb29-10f9-4b3fa7b2bbb2@2ndquadrant.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/dcdbb5a5db09064ac08ff3971c5031281ef2e545

Modified Files
--------------
doc/src/sgml/logicaldecoding.sgml | 6 +++---
doc/src/sgml/ref/psql-ref.sgml | 2 +-
src/bin/psql/settings.h | 4 ++--
3 files changed, 6 insertions(+), 6 deletions(-)