process startup: Always call Init[Auxiliary]Process() before BaseInit().

For EXEC_BACKEND InitProcess()/InitAuxiliaryProcess() needs to have been
called well before we call BaseInit(), as SubPostmasterMain() needs LWLocks to
work. Having the order of initialization differ between platforms makes it
unnecessarily hard to understand the system and to add initialization points
for new subsystems without a lot of duplication.

To be able to change the order, BaseInit() cannot trigger
CreateSharedMemoryAndSemaphores() anymore - obviously that needs to have
happened before we can call InitProcess(). It seems cleaner to create shared
memory explicitly in single user/bootstrap mode anyway.

After this change the separation of bufmgr initialization into
InitBufferPoolAccess() / InitBufferPoolBackend() is not meaningful anymore so
the latter is removed.

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

src/backend/postmaster/auxprocess.c

@@ -90,8 +90,6 @@ AuxiliaryProcessMain(AuxProcType auxtype)
 	SetProcessingMode(BootstrapProcessing);
 	IgnoreSystemIndexes = true;
 
-	BaseInit();
-
 	/*
 	 * As an auxiliary process, we aren't going to do the full InitPostgres
 	 * pushups, but there are a couple of things that need to get lit up even
@@ -106,6 +104,8 @@ AuxiliaryProcessMain(AuxProcType auxtype)
 	InitAuxiliaryProcess();
 #endif
 
+	BaseInit();
+
 	/*
 	 * Assign the ProcSignalSlot for an auxiliary process.  Since it doesn't
 	 * have a BackendId, the slot is statically allocated based on the
@@ -118,9 +118,6 @@ AuxiliaryProcessMain(AuxProcType auxtype)
 	 */
 	ProcSignalInit(MaxBackends + MyAuxProcType + 1);
 
-	/* finish setting up bufmgr.c */
-	InitBufferPoolBackend();
-
 	/*
 	 * Auxiliary processes don't run transactions, but they may need a
 	 * resource owner anyway to manage buffer pins acquired outside