pg_dump additional options for performance

Simon,

I agree with adding these options in general, since I find myself
frustrated by having to vi huge dumps to change simple schema things.
A couple of comments on the patch though:

- Conflicting option handling
I think we are doing our users a disservice by putting it on them to
figure out exactly what:
multiple object groups cannot be used together
means to them. You and I may understand what an "object group" is,
and why there can be only one, but it's a great deal less clear than
the prior message of
options -s/--schema-only and -a/--data-only cannot be used together
My suggestion would be to either list out the specific options which
can't be used together, as was done previously, or add a bit of (I
realize, boring) code and actually tell the user which of the
conflicting options were used.

- Documentation
When writing the documentation I would stress that "pre-schema" and
"post-schema" be defined in terms of PostgreSQL objects and why they
are pre vs. post.

- Technically, the patch needs to be updated slightly since another
pg_dump-related patch was committed recently which also added
options and thus causes a conflict.

Beyond those minor points, the patch looks good to me.

Thanks,

Stephen