Revise parse tree representation for VACUUM and ANALYZE.

Revise parse tree representation for VACUUM and ANALYZE.

Like commit f41551f61f9cf4eedd5b7173f985a3bdb4d9858c, this aims
to make it easier to add non-Boolean options to VACUUM (or, in
this case, to ANALYZE).  Instead of building up a bitmap of
options directly in the parser, build up a list of DefElem
objects and let ExecVacuum() sort it out; right now, we make
no use of the fact that a DefElem can carry an associated value,
but it will be easy to make that change in the future.

Masahiko Sawada

Discussion: http://postgr.es/m/CAD21AoATE4sn0jFFH3NcfUZXkU2BMbjBWB_kDj-XWYA-LXDcQA@mail.gmail.com