Check whether is_client can be replaced by is_top_level_client

[egrimley-arm]

It is currently unclear whether is_client is or should be set for extension libraries as well as top-level clients. The code should be audited to see exactly how that flag is used, and perhaps is_client and is_top_level_client could be merged.

There is an attempt to set is_client in privload_process_imports() but the condition impmod->base == get_dynamorio_dll_start() is never true at that point because the check is inside if (privload_lookup(name) == NULL) and libdynamorio.so is already loaded.

See #6976.