Fix advertising autovacuum launcher's ProcNumber

Fix advertising autovacuum launcher's ProcNumber

Autovacuum launcher is not an aux process, so it needs to be
advertised in InitProcess() rather than InitAuxiliaryProcess()

In the passing, also remove some now-unnecessary 'volatile' qualifiers
left behind by the same commit.

Reported-by: Thom Brown <thom@linux.com>
Author: Fujii Masao <masao.fujii@gmail.com>
Author: Nathan Bossart <nathandbossart@gmail.com>
Discussion: https://www.postgresql.org/message-id/CAA-aLv6UHXubNxmAjNH8PnyKKkaXePO1ggB15=iidNW08T4hSg@mail.gmail.com