Sort guc_parameters.dat alphabetically by name

 Sort guc_parameters.dat alphabetically by name
 
 The order in this list was previously pretty random and had grown
 organically over time.  This made it unnecessarily cumbersome to
 maintain these lists, as there was no clear guidelines about where to
 put new entries.  Also, after the merger of the type-specific GUC
 structs, the list still reflected the previous type-specific
 super-order.
 
 By using alphabetical order, the place for new entries becomes clear,
 and often related entries will be listed close together.
 
 This patch reorders the existing entries in guc_parameters.dat, and it
 also augments the generation script to error if an entry is found at
 the wrong place.
 
 Note: The order is actually checked after lower-casing, to handle the
 likes of "DateStyle".
 
 Reviewed-by: John Naylor <johncnaylorls@gmail.com>
 Reviewed-by: Álvaro Herrera <alvherre@kurilemu.de>
 Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
 Discussion: https://www.postgresql.org/message-id/flat/8fdfb91e-60fb-44fa-8df6-f5dea47353c9@eisentraut.org