psql: Add more information about service name
 
This commit adds support for the following items in psql, able to show a
service name, when available:
- Variable SERVICE.
- Substitution %s in PROMPT{1,2,3}.
 
This relies on 4b99fed7541e, that has made the service name available in
PGconn for libpq.
 
Author: Michael Banck
Reviewed-by: Greg Sabino Mullane
Discussion: https://postgr.es/m/6723c612.050a0220.1567f4.b94a@mx.google.com