pgsql: Add option list to CHECKPOINT command.

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(at)debian(dot)org>
Reviewed-by: Fujii Masao <masao(dot)fujii(at)oss(dot)nttdata(dot)com>
Discussion: https://postgr.es/m/aDnaKTEf-0dLiEfz%40msg.df7cb.de

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/a4f126516e688736bfed332b44a0c221b8dc118a

Modified Files
--------------
doc/src/sgml/ref/checkpoint.sgml | 11 ++++++++++-
src/backend/parser/gram.y | 7 +++++++
src/backend/postmaster/checkpointer.c | 31 +++++++++++++++++++++++++++++++
src/backend/tcop/utility.c | 12 +-----------
src/bin/psql/tab-complete.in.c | 3 +++
src/include/nodes/parsenodes.h | 1 +
src/include/postmaster/bgwriter.h | 2 ++
src/test/regress/expected/stats.out | 6 ++++++
src/test/regress/sql/stats.sql | 3 +++
9 files changed, 64 insertions(+), 12 deletions(-)