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/walwriter.c

@@ -48,6 +48,7 @@
 #include "libpq/pqsignal.h"
 #include "miscadmin.h"
 #include "pgstat.h"
+#include "postmaster/auxprocess.h"
 #include "postmaster/interrupt.h"
 #include "postmaster/walwriter.h"
 #include "storage/bufmgr.h"
@@ -85,13 +86,18 @@ int			WalWriterFlushAfter = DEFAULT_WAL_WRITER_FLUSH_AFTER;
  * basic execution environment, but not enabled signals yet.
  */
 void
-WalWriterMain(void)
+WalWriterMain(char *startup_data, size_t startup_data_len)
 {
 	sigjmp_buf	local_sigjmp_buf;
 	MemoryContext walwriter_context;
 	int			left_till_hibernate;
 	bool		hibernating;
 
+	Assert(startup_data_len == 0);
+
+	MyBackendType = B_WAL_WRITER;
+	AuxiliaryProcessMainCommon();
+
 	/*
 	 * Properly accept or ignore signals the postmaster might send us
 	 *