Add option list to CHECKPOINT command.

This commit adds the boilerplate code for supporting a list of
options in CHECKPOINT commands.  No actual options are supported
yet, but follow-up commits will add support for MODE and
FLUSH_UNLOGGED.  While at it, this commit refactors the code for
executing CHECKPOINT commands to its own function since it's about
to become significantly larger.

Author: Christoph Berg <myon@debian.org>
Reviewed-by: Fujii Masao <masao.fujii@oss.nttdata.com>
Discussion: https://postgr.es/m/aDnaKTEf-0dLiEfz%40msg.df7cb.de

src/backend/parser/gram.y

@@ -2034,6 +2034,13 @@ CheckPointStmt:
 
 					$$ = (Node *) n;
 				}
+			| CHECKPOINT '(' utility_option_list ')'
+				{
+					CheckPointStmt *n = makeNode(CheckPointStmt);
+
+					$$ = (Node *) n;
+					n->options = $3;
+				}
 		;