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/utils/init/globals.c

@@ -45,6 +45,7 @@ volatile uint32 CritSectionCount = 0;
 int			MyProcPid;
 pg_time_t	MyStartTime;
 TimestampTz MyStartTimestamp;
+struct ClientSocket *MyClientSocket;
 struct Port *MyProcPort;
 int32		MyCancelKey;
 int			MyPMChildSlot;