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/test/regress/expected/stats.out

@@ -926,6 +926,12 @@ DROP TABLE test_stats_temp;
 -- Checkpoint twice: The checkpointer reports stats after reporting completion
 -- of the checkpoint. But after a second checkpoint we'll see at least the
 -- results of the first.
+--
+-- While at it, test checkpoint options.
+CHECKPOINT (WRONG);
+ERROR:  unrecognized CHECKPOINT option "wrong"
+LINE 1: CHECKPOINT (WRONG);
+                    ^
 CHECKPOINT;
 CHECKPOINT;
 SELECT num_requested > :rqst_ckpts_before FROM pg_stat_checkpointer;

src/test/regress/sql/stats.sql

@@ -439,6 +439,9 @@ DROP TABLE test_stats_temp;
 -- Checkpoint twice: The checkpointer reports stats after reporting completion
 -- of the checkpoint. But after a second checkpoint we'll see at least the
 -- results of the first.
+--
+-- While at it, test checkpoint options.
+CHECKPOINT (WRONG);
 CHECKPOINT;
 CHECKPOINT;