Support for various queries sent by pgpool2

* Allow setting application_name. This is normally initialized at connection time, but it's harmless to change it. Perhaps we should display it in some system table or the thread name. But nothing like that is included here.

* Various pg_ queries that return 0 as the count of special tables. This is from its query analysis to determine what needs to be sent to the master. Since we don't have a master in our configuration, all queries are safe, so nothing is special.

* The most substantial change in terms of number of files affected. Expose a current_setting function for getting a setting. This can be something set via the SET command, or something initialized by the Postgres connect dialog. And for pgpool, this can be transaction_isolation, for which we claim serializable. All this is more or less compatible with Postgres.

* The previous means that almost all of the SHOW command is now wired up, except for the actual syntax. I am interesting in opinions on whether we ought to support it.