pgsql: Allow psql to re-use connection parameters after a connection lo

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/1b62d0fb3e50ede570d0d4e4a2be69d5645b48a7

Modified Files
--------------
doc/src/sgml/ref/psql-ref.sgml | 17 ++++++++++---
src/bin/psql/command.c         | 57 +++++++++++++++++++++++++++++-------------
src/bin/psql/common.c          | 10 +++++---
src/bin/psql/describe.c        |  2 +-
src/bin/psql/settings.h        |  7 ++++++
src/bin/psql/startup.c         |  6 ++++-
6 files changed, 73 insertions(+), 26 deletions(-)