Fix mishandling of background worker PGPROCs in EXEC_BACKEND builds.

InitProcess() relies on IsBackgroundWorker to decide whether the PGPROC
for a new backend should be taken from ProcGlobal's freeProcs or from
bgworkerFreeProcs.  In EXEC_BACKEND builds, InitProcess() is called
sooner than in non-EXEC_BACKEND builds, and IsBackgroundWorker wasn't
getting initialized soon enough.

Report by Noah Misch.  Diagnosis and fix by me.

src/backend/postmaster/postmaster.c

@@ -4566,6 +4566,9 @@ SubPostmasterMain(int argc, char *argv[])
 	{
 		int			cookie;
 
+		/* do this as early as possible; in particular, before InitProcess() */
+		IsBackgroundWorker = true;
+
 		/* Close the postmaster's sockets */
 		ClosePostmasterPorts(false);