Refactor parsing rules for option lists of EXPLAIN, VACUUM and ANALYZE

Refactor parsing rules for option lists of EXPLAIN, VACUUM and ANALYZE

Those three commands have been using the same grammar rules to handle a
a list of parenthesized options.  This refactors the code so as they use
the same parsing rules, shaving some code.  A future commit will make
use of those option parsing rules for more utility commands, like
REINDEX and CLUSTER.

Author: Alexey Kondratov, Justin Pryzby
Discussion: https://postgr.es/m/8a8f5f73-00d3-55f8-7583-1375ca8f6a91@postgrespro.ru