pgsql: psql: Add variable SERVICEFILE

pgsql: psql: Add variable SERVICEFILE

psql: Add variable SERVICEFILE

This new psql variable can be used to check which service file has been
used for a connection.  Like other variables, this can be set in a
PROMPT or reported by an \echo, like these commands:
\echo :SERVICEFILE
\set PROMPT1 '=(%:SERVICEFILE:)%# '

This relies on commits 092f3c63efc6 and fef6da9e9c87 to retrieve this
information from the connection's PQconninfoOption.

Author: Ryo Kanbayashi 
Discussion: https://postgr.es/m/CAKkG4_nCjx3a_F3gyXHSPWxD8Sd8URaM89wey7fG_9g7KBkOCQ@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/6b1c4d326b064bf0eaedccb08a7fcca5db5d9629

Modified Files
--------------
doc/src/sgml/ref/psql-ref.sgml | 9 +++++++++
src/bin/psql/command.c         | 7 +++++++
2 files changed, 16 insertions(+)