Fix psql's \connect command some more.

Jasen Betts reported yet another unintended side effect of commit
85c54287: reconnecting with "\c service=whatever" did not have the
expected results.  The reason is that starting from the output of
PQconndefaults() effectively allows environment variables (such
as PGPORT) to override entries in the service file, whereas the
normal priority is the other way around.

Not using PQconndefaults at all would require yet a third main code
path in do_connect's parameter setup, so I don't really want to fix
it that way.  But we can have the logic effectively ignore all the
default values for just a couple more lines of code.

This patch doesn't change the behavior for "\c -reuse-previous=on
service=whatever".  That remains significantly different from before
85c54287, because many more parameters will be re-used, and thus
not be possible for service entries to replace.  But I think this
is (mostly?) intentional.  In any case, since libpq does not report
where it got parameter values from, it's hard to do differently.

Per bug #16936 from Jasen Betts.  As with the previous patches,
back-patch to all supported branches.  (9.5 is unfortunately now
out of support, so this won't get fixed there.)

Discussion: https://postgr.es/m/16936-3f524322a53a29f0@postgresql.org

src/bin/psql/command.c

@@ -3132,6 +3132,25 @@ do_connect(enum trivalue reuse_previous_specification,
 						if (strcmp(replci->keyword, "password") == 0)
 							have_password = true;
 					}
+					else if (!reuse_previous)
+					{
+						/*
+						 * When we have a connstring and are not re-using
+						 * parameters, swap *all* entries, even those not set
+						 * by the connstring.  This avoids absorbing
+						 * environment-dependent defaults from the result of
+						 * PQconndefaults().  We don't want to do that because
+						 * they'd override service-file entries if the
+						 * connstring specifies a service parameter, whereas
+						 * the priority should be the other way around.  libpq
+						 * can certainly recompute any defaults we don't pass
+						 * here.  (In this situation, it's a bit wasteful to
+						 * have called PQconndefaults() at all, but not doing
+						 * so would require yet another major code path here.)
+						 */
+						replci->val = ci->val;
+						ci->val = NULL;
+					}
 				}
 				Assert(ci->keyword == NULL && replci->keyword == NULL);