psql's \connect claims to be able to re-use previous connection
parameters, but in fact it only re-uses the database name, user name,
host name (and possibly hostaddr, depending on version), and port.
This is problematic for assorted use cases.  Notably, pg_dump[all]
emits "\connect databasename" commands which we would like to have
re-use all other parameters.  If such a script is loaded in a psql run
that initially had "-d connstring" with some non-default parameters,
those other parameters would be lost, potentially causing connection
failure.  (Thus, this is the same kind of bug addressed in commits
a45bc8a4 and 8e5793ab, although the details are much different.)

To fix, redesign do_connect() so that it pulls out all properties
of the old PGconn using PQconninfo(), and then replaces individual
properties in that array.  In the case where we don't wish to re-use
anything, get libpq's default settings using PQconndefaults() and
replace entries in that, so that we don't need different code paths
for the two cases.

This does result in an additional behavioral change for cases where
the original connection parameters allowed multiple hosts, say
"psql -h host1,host2", and the \connect request allows re-use of the
host setting.  Because the previous coding relied on PQhost(), it
would only permit reconnection to the same host originally selected.
Although one can think of scenarios where that's a good thing, there
are others where it is not.  Moreover, that behavior doesn't seem to
meet the principle of least surprise, nor was it documented; nor is
it even clear it was intended, since that coding long pre-dates the
addition of multi-host support to libpq.  Hence, this patch is content
to drop it and re-use the host list as given.

Per Peter Eisentraut's comments on bug #16604.  Back-patch to all
supported branches.

Discussion: https://postgr.es/m/16604-933f4b8791227b15@postgresql.org