Refactor ReindexStmt and its "concurrent" boolean

Hi all,

$subject has been mentioned a couple of times, including today:
https://www.postgresql.org/message-id/20200902010012.GE1489@paquier.xyz

We have a boolean argument in ReindexStmt to control a concurrent
run, and we also have in parallel of that a bitmask to control the
options of the statement, which feels like a duplicate.  Attached is a
patch to refactor the whole, adding CONCURRENTLY as a member of the
available options.  This simplifies a bit the code.

Any thoughts?
--
Michael