Fix up pg_dump to emit shell-type definitions at the proper time, to

make use of the recently added ability to create a shell type explicitly.

I also put in place some infrastructure to allow dump/no dump decisions
to be made separately for each database object, rather than the former
hardwired 'dump if in a dumpable schema' policy.  This was needed anyway
for shell types so now seemed a convenient time to do it.  The flexibility
isn't exposed to the user yet, but is ready for future extensions.