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
0ea9efbe.

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.