Pass BackgroundWorker entry in the parameter file in EXEC_BACKEND mode

Pass BackgroundWorker entry in the parameter file in EXEC_BACKEND mode

The BackgroundWorker struct is now passed the same way as the Port
struct. Seems more consistent.

This makes it possible to move InitProcess later in SubPostmasterMain
(in next commit), as we no longer need to access shared memory to read
background worker entry.

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