This is a necessary preliminary step for making it play with psqlscan.l
given the way I set up the lexer input-buffer sharing mechanism in commit
0ea9efbe9e.
I've not tried to make it *actually* reentrant; there's still some static
variables laying about. But flex thinks it's reentrant, and that's what
counts.
In support of that, fix exprparse.y to pass through the yyscan_t from the
caller. Also do some minor code beautification, like not casting away
const.
Fabien Coelho, reviewed mostly by Michael Paquier and me, but also by
Heikki Linnakangas, BeomYong Lee, Kyotaro Horiguchi, Oleksander
Shulgin, and Álvaro Herrera.
