Set the stack_base_ptr in main(), not in random other places.

Previously we did this in PostmasterMain() and InitPostmasterChild(),
which meant that stack depth checking was disabled in non-postmaster
server processes, for instance in single-user mode.  That seems like
a fairly bad idea, since there's no a-priori restriction on the
complexity of queries we will run in single-user mode.  Moreover, this
led to not having quite the same stack depth limit in all processes,
which likely has no real-world effect but it offends my inner neatnik.
Setting the depth in main() guarantees that check_stack_depth() is
armed and has a consistent interpretation of stack depth in all forms
of server processes.

While at it, move the code associated with checking the stack depth
out of tcop/postgres.c (which was never a great home for it) into
a new file src/backend/utils/misc/stack_depth.c.

Discussion: https://postgr.es/m/2081982.1734393311@sss.pgh.pa.us

src/backend/postmaster/postmaster.c

@@ -985,11 +985,6 @@ PostmasterMain(int argc, char *argv[])
 	 */
 	set_max_safe_fds();
 
-	/*
-	 * Set reference point for stack-depth checking.
-	 */
-	(void) set_stack_base();
-
 	/*
 	 * Initialize pipe (or process handle on Windows) that allows children to
 	 * wake up from sleep on postmaster death.