Field conninfo strings throughout src/bin/scripts.

These programs nominally accepted conninfo strings, but they would
proceed to use the original dbname parameter as though it were an
unadorned database name.  This caused "reindexdb dbname=foo" to issue an
SQL command that always failed, and other programs printed a conninfo
string in error messages that purported to print a database name.  Fix
both problems by using PQdb() to retrieve actual database names.
Continue to print the full conninfo string when reporting a connection
failure.  It is informative there, and if the database name is the sole
problem, the server-side error message will include the name.  Beyond
those user-visible fixes, this allows a subsequent commit to synthesize
and use conninfo strings without that implementation detail leaking into
messages.  As a side effect, the "vacuuming database" message now
appears after, not before, the connection attempt.  Back-patch to 9.1
(all supported versions).

Reviewed by Michael Paquier and Peter Eisentraut.

Security: CVE-2016-5424

src/bin/scripts/createlang.c

@@ -192,10 +192,10 @@ main(int argc, char *argv[])
 	result = executeQuery(conn, sql.data, progname, echo);
 	if (PQntuples(result) > 0)
 	{
-		PQfinish(conn);
 		fprintf(stderr,
 		  _("%s: language \"%s\" is already installed in database \"%s\"\n"),
-				progname, langname, dbname);
+				progname, langname, PQdb(conn));
+		PQfinish(conn);
 		/* separate exit status for "already installed" */
 		exit(2);
 	}