BUG #16936: psql: \c service= not working

The following bug has been logged on the website:

Bug reference: 16936
Logged by: Jasen Betts
Email address: pg_bugs(at)revmaps(dot)no-ip(dot)org
PostgreSQL version: 13.2
Operating system: Linux Debian 10.8 on amd64
Description:

The \c metacommand in psql does not anymore use the full information from
pg_service.conf

It's reading the pg_service.conf file but seems to ignore everything but the
dbname=part part of the entry. this bug seems to be new, but is possibly not
in the psql binary itself, as the bug seems to have at-least partially
migrated to older versions of psql too.

The following is a verbatim transcript except for the initial secret
connection string. obviously the service entry for "[j]" is broken on every
line except dbname, yet all those lines are being ignored.

jasen(at)ryan:~$ psql 'secret-connection-string deleted'
psql (13.2 (Debian 13.2-1.pgdg100+1), server 10.16 (Debian
10.16-1.pgdg90+1))
SSL connection (protocol: TLSv1.2, cipher: ECDHE-RSA-AES256-GCM-SHA384,
bits: 256, compression: off)
Type "help" for help.

back2=> \c service=j
psql (13.2 (Debian 13.2-1.pgdg100+1), server 11.10 (Debian
11.10-1.pgdg100+1))
You are now connected to database "jasen" as user "jasen" via socket in
"/var/run/postgresql" at port "5432".
jasen=# \q
jasen(at)ryan:~$ psql service=j
psql: error: received invalid response to SSL negotiation: H
jasen(at)ryan:~$ cat .pg_service.conf
[j]
host=example.com
dbname=jasen
user=nobody
password=paternal_gladius
port=80

jasen(at)ryan:~$