process startup: Do InitProcess() at the same time regardless of EXEC_BACKEND.

An upcoming patch splits single user mode into its own function. This makes
that easier. Split out for easier review / testing.

Reviewed-By: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Author: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/20210802164124.ufo5buo4apl6yuvs@alap3.anarazel.de

src/backend/postmaster/postmaster.c

@@ -4275,6 +4275,15 @@ BackendStartup(Port *port)
 		/* Perform additional initialization and collect startup packet */
 		BackendInitialize(port);
 
+		/*
+		 * Create a per-backend PGPROC struct in shared memory. We must do
+		 * this before we can use LWLocks. In the !EXEC_BACKEND case (here)
+		 * this could be delayed a bit further, but EXEC_BACKEND needs to do
+		 * stuff with LWLocks before PostgresMain(), so we do it here as well
+		 * for symmetry.
+		 */
+		InitProcess();
+
 		/* And run the backend */
 		BackendRun(port);
 	}