libpq should expose GSS-related parameters even when not implemented.

We realized years ago that it's better for libpq to accept all
connection parameters syntactically, even if some are ignored or
restricted due to lack of the feature in a particular build.
However, that lesson from the SSL support was for some reason never
applied to the GSSAPI support.  This is causing various buildfarm
members to have problems with a test case added by commit 6136e94dc,
and it's just a bad idea from a user-experience standpoint anyway,
so fix it.

While at it, fix some places where parameter-related infrastructure
was added with the aid of a dartboard, or perhaps with the aid of
the anti-pattern "add new stuff at the end".  It should be safe
to rearrange the contents of struct pg_conn even in released
branches, since that's private to libpq (and we'd have to move
some fields in some builds to fix this, anyway).

Back-patch to all supported branches.

Discussion: https://postgr.es/m/11297.1576868677@sss.pgh.pa.us

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -134,8 +134,8 @@ ALTER TABLE ft_pg_type SET WITH OIDS;
 -- ===================================================================
 -- tests for validator
 -- ===================================================================
--- requiressl, krbsrvname and gsslib are omitted because they depend on
--- configure options
+-- requiressl and some other parameters are omitted because
+-- valid values for them depend on configure options
 ALTER SERVER testserver1 OPTIONS (
 	use_remote_estimate 'false',
 	updatable 'true',
@@ -159,10 +159,10 @@ ALTER SERVER testserver1 OPTIONS (
 	sslcert 'value',
 	sslkey 'value',
 	sslrootcert 'value',
-	sslcrl 'value'
+	sslcrl 'value',
 	--requirepeer 'value',
-	-- krbsrvname 'value',
-	-- gsslib 'value',
+	krbsrvname 'value',
+	gsslib 'value'
 	--replication 'value'
 );
 -- Error, invalid list syntax