Don't dump Qunbound

Don't dump Qunbound

The dumper uses a hash table to keep track of dumped objects but as
this clashes with the use of Qunbound for marking unused hash table
entries, don't dump that value at all.  The symbol name is fixed up
after loading.

An alternative solution would be to use a different unique value for
unused entries.

* src/pdumper.c (dump_object_needs_dumping_p): Skip Qunbound.
(dump_vectorlike_generic): New function.
(pdumper_load): Call it.

(cherry picked from commit 7ad5d427730fea3865bc678c6673ffd58b6af653)