Allow psql to re-use connection parameters after a connection loss.

Instead of immediately PQfinish'ing a dead connection, save it aside
so that we can still extract its parameters for \connect attempts.
(This works because PQconninfo doesn't care whether the PGconn is in
CONNECTION_BAD state.)  This allows developers to reconnect with
just \c after a database crash and restart.

It's tempting to use the same approach instead of closing the old
connection after a failed non-interactive \connect command.  However,
that would not be very safe: consider a script containing
	\c db1 user1 live_server
	\c db2 user2 dead_server
	\c db3
The script would be expecting to connect to db3 at dead_server, but
if we re-use parameters from the first connection then it might
successfully connect to db3 at live_server.  This'd defeat the goal
of not letting a script accidentally execute commands against the
wrong database.

Discussion: https://postgr.es/m/38464.1603394584@sss.pgh.pa.us

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