dollar quoting with flex

(Fourth try ;-)

Attached is a patch for dollar quoting in the backend and in psql (with
the new flex scanner). I'm fairly confident about the backend (because
this is mainly Tom's work adapted :-) ) but rather less so about psql -
I don't entirely understand all the odd states in psql's scanner. I'm
not sure that I have freed up memory in all the necessary cases. Nor am
I sure what the state is or should be if we end an included file in a
dollar-quoting state, nor how to handle such a situation. So, some extra
eyeballs would be appreciated.

However - it does seem to work in my simple testing.

If this is all OK, the remaining tasks would include pg_dump, docs (Jon
Jensen says he will attack these two) and some regression tests (any
volunteers?)

cheers

andrew