Replace postmaster.c's own backend type codes with BackendType

Introduce a separate BackendType for dead-end children, so that we
don't need a separate dead_end flag.

Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://www.postgresql.org/message-id/a102f15f-eac4-4ff2-af02-f9ff209ec66f@iki.fi

Author: hlinnaka

src/backend/postmaster/launch_backend.c

@@ -174,6 +174,7 @@ static child_process_kind child_process_kinds[] = {
 	[B_INVALID] = {"invalid", NULL, false},
 
 	[B_BACKEND] = {"backend", BackendMain, true},
+	[B_DEAD_END_BACKEND] = {"dead-end backend", BackendMain, true},
 	[B_AUTOVAC_LAUNCHER] = {"autovacuum launcher", AutoVacLauncherMain, true},
 	[B_AUTOVAC_WORKER] = {"autovacuum worker", AutoVacWorkerMain, true},
 	[B_BG_WORKER] = {"bgworker", BackgroundWorkerMain, true},