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 <[email protected]>
Discussion: https://postgr.es/m/CAKkG4_nCjx3a_F3gyXHSPWxD8Sd8URaM89wey7fG_9g7KBkOCQ@mail.gmail.com