Refactor how InitProcess is called

Refactor how InitProcess is called

The order of process initialization steps is now more consistent
between !EXEC_BACKEND and EXEC_BACKEND modes. InitProcess() is called
at the same place in either mode. We can now also move the
AttachSharedMemoryStructs() call into InitProcess() itself. This
reduces the number of "#ifdef EXEC_BACKEND" blocks.

Reviewed-by: Tristan Partin, Andres Freund, Alexander Lakhin
Discussion: https://www.postgresql.org/message-id/7a59b073-5b5b-151e-7ed3-8b01ff7ce9ef@iki.fi