guc: reentrant scanner

guc: reentrant scanner

Use the flex %option reentrant to make the generated scanner
reentrant, and perhaps eventually thread-safe, but that will require
additional work.

Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Reviewed-by: Andreas Karlsson <andreas@proxel.se>
Discussion: https://www.postgresql.org/message-id/flat/eb6faeac-2a8a-4b69-9189-c33c520e5b7b@eisentraut.org