Re: Concurrent psql patch

So based on the feedback and suggestions here this is the interface I suggest:

\connect& - to open a new connection keeping the existing one
\g& - to submit a command asynchronously (like & in the shell)
\S [Sess#] - to _S_witch to a different _S_ession
- if no connection # specified list available _S_essions
\D - _D_isconnect from current session (like ^D in the shell)

This leaves no way to submit an asynchronous command without using \g but I'm
really not too concerned with that. I don't want to start messing with psql's
semicolon parsing behaviour and I'm mainly only concerned with this for
regression tests.

Another thought I had for the future is a \C command to simulate C-c and send
a query cancel. That would let us have regression tests that query
cancellation worked. The tests would presumably have to be written using
pg_sleep() to ensure they ran for long enough but even then there would be no
way to control exactly when the interrupt arrived.

Attached is an updated patch.

I also found and fixed some missing ResetCancelConn()s. I think I got them all
and the behaviour seems correct in practice when cancelling various
combinations of synchronous queries, asynchronous queries, and backslash
commands. The one thing I wonder about is that I'm a bit concerned I may have
introduced an assumption about how many resultsets arrive from a single query.

I'll be offline for a few days but I'll be back Monday.