generic explain options v2

Here's an updated version of my "generic options for explain" patch.

http://archives.postgresql.org/message-id/603c8f070905231747j2e099c23hef8eafbf26682e5f@mail.gmail.com

This is rebased to CVS HEAD, post pgindent. Also, I've made a few
changes to the syntax. Previously, I suggested this syntax:

explain (analyze 'on', verbose 'off') query...

Greg Stark suggested that for boolean parameters we might allow the
argument to be omitted and default to true.

http://archives.postgresql.org/message-id/4C72394C-384F-4FC4-BE3E-8F556FBD3E4B@enterprisedb.com

That seemed like a good idea so I did it. I also took it a step
further and did something we've done elsewhere in the parser so that
in many cases the quotes aren't even necessary. Thus you can now also
write:

explain (analyze on, verbose off) query...

Of course, we don't have any consensus on the right syntax, but my
previous proposal had at least as much support as any of the other
alternatives that were offered, so we'll see where we get with this
one.

Followup patch that adds actual new functionality coming shortly.

...Robert