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/pgarch.c

@@ -36,6 +36,7 @@
 #include "lib/binaryheap.h"
 #include "libpq/pqsignal.h"
 #include "pgstat.h"
+#include "postmaster/auxprocess.h"
 #include "postmaster/interrupt.h"
 #include "postmaster/pgarch.h"
 #include "storage/fd.h"
@@ -209,8 +210,13 @@ PgArchCanRestart(void)
 
 /* Main entry point for archiver process */
 void
-PgArchiverMain(void)
+PgArchiverMain(char *startup_data, size_t startup_data_len)
 {
+	Assert(startup_data_len == 0);
+
+	MyBackendType = B_ARCHIVER;
+	AuxiliaryProcessMainCommon();
+
 	/*
 	 * Ignore all signals usually bound to some action in the postmaster,
 	 * except for SIGHUP, SIGTERM, SIGUSR1, SIGUSR2, and SIGQUIT.