\timing on/off

I started to look at David Fetter's patch to enable "\timing on/off", in
addition to toggling the mod with just "\timing".

I gather that the conclusion of the thread "Making sure \timing is on",
starting at:
http://archives.postgresql.org/pgsql-general/2008-05/msg00324.php

was that we should leave \H and \a alone for now, because it's not clear
what "\H off" would do. And \a is already documented as deprecated; we
might want to do that for \H as well.

Here's a patch, based on David's patch, that turns timing into a \pset
variable, and makes \timing an alias for \pset timing. This makes
\timing behave the same as \x.

I also moved the help line from the "External" section into
"Formatting". I don't think \timing is "external" in the same sense as
\cd or \! are. Rather, it controls the output, like \x or \pset options.

--
   Heikki Linnakangas
   EnterpriseDB   http://www.enterprisedb.com