Refactor postmaster child process launching

Introduce new postmaster_child_launch() function that deals with the
differences in EXEC_BACKEND mode.

Refactor the mechanism of passing information from the parent to child
process. Instead of using different command-line arguments when
launching the child process in EXEC_BACKEND mode, pass a
variable-length blob of startup data along with all the global
variables. The contents of that blob depend on the kind of child
process being launched. In !EXEC_BACKEND mode, we use the same blob,
but it's simply inherited from the parent to child process.

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

src/backend/postmaster/startup.c

@@ -24,6 +24,7 @@
 #include "access/xlogutils.h"
 #include "libpq/pqsignal.h"
 #include "miscadmin.h"
+#include "postmaster/auxprocess.h"
 #include "postmaster/startup.h"
 #include "storage/ipc.h"
 #include "storage/pmsignal.h"
@@ -212,8 +213,13 @@ StartupProcExit(int code, Datum arg)
  * ----------------------------------
  */
 void
-StartupProcessMain(void)
+StartupProcessMain(char *startup_data, size_t startup_data_len)
 {
+	Assert(startup_data_len == 0);
+
+	MyBackendType = B_STARTUP;
+	AuxiliaryProcessMainCommon();
+
 	/* Arrange to clean up at startup process exit */
 	on_shmem_exit(StartupProcExit, 0);