The dialog hardcoded the pgpass flag to false, so it always demanded the
current password even when the connection authenticates via a passfile.
Pass the flag from check_pgpass through to the schema instead.
check_pgpass and change_password also disagreed about when a passfile is
in use: the former treated a password entered at connect but not saved
as a passfile connection, which would now disable the Current Password
field whilst the endpoint still required it. Both now use a shared
is_using_passfile() helper.
Fixes#10449
bdd3282c49