Fix up dumping conditions for extension configuration tables.

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 7b070e896ca835318c90b02c830a5c4844413b64, 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.