pg_dump: Rename some typedefs to avoid name conflicts

pg_dump: Rename some typedefs to avoid name conflicts

In struct _archiveHandle, some of the fields have the same name as a
typedef.  This is kind of confusing, so rename the types so they have
names distinct from the struct fields.  In C++, the previous coding
changes the meaning of the typedef in the scope of the struct, causing
warnings and possibly other problems.

Reviewed-by: Andres Freund <andres@anarazel.de>