Replace the sorted array of GUC variables with a hash table.

This gets rid of bsearch() in favor of hashed lookup.  The main
advantage is that it becomes far cheaper to add new GUCs, since
we needn't re-sort the pointer array.  Adding N new GUCs had
been O(N^2 log N), but now it's closer to O(N).  We need to
sort only in SHOW ALL and equivalent functions, which are
hopefully not performance-critical to anybody.

Also, merge GetNumConfigOptions() into get_guc_variables(),
because in a world where the set of GUCs isn't fairly static
you really want to consider those two results as tied together
not independent.

Discussion: https://postgr.es/m/2982579.1662416866@sss.pgh.pa.us

src/backend/utils/misc/help_config.c

@@ -49,11 +49,10 @@ GucInfoMain(void)
 	int			numOpts,
 				i;
 
-	/* Initialize the guc_variables[] array */
+	/* Initialize the GUC hash table */
 	build_guc_variables();
 
-	guc_vars = get_guc_variables();
-	numOpts = GetNumConfigOptions();
+	guc_vars = get_guc_variables(&numOpts);
 
 	for (i = 0; i < numOpts; i++)
 	{