Fix up dumping conditions for extension configuration tables.

Various filters that were meant to prevent dumping of table data were not
being applied to extension config tables, notably --exclude-table-data and
--no-unlogged-table-data.  We also would bogusly try to dump data from
views, sequences, or foreign tables, should an extension try to claim they
were config tables.  Fix all that, and refactor/redocument to try to make
this a bit less fragile.  This reverts the implementation, though not the
feature, of commit 7b070e89, which had
broken config-table dumping altogether :-(.

It is still the case that the code will dump config-table data even if
--schema is specified.  That behavior was intentional, as per the comments
in getExtensionMembership, so I think it requires some more discussion
before we change it.