pgsql: Add option -N/--no-sync to pg_checksums

Add option -N/--no-sync to pg_checksums

This is an option consistent with what pg_dump, pg_rewind and
pg_basebackup provide which is useful for leveraging the I/O effort when
testing things, not to be used in a production environment.

Author: Michael Paquier
Reviewed-by: Michael Banck, Fabien Coelho, Sergei Kornilov
Discussion: https://postgr.es/m/20181221201616.GD4974@nighthawk.caipicrew.dd-dns.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e0090c86900877bf0911c53dcf4a30bc81d03047

Modified Files
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doc/src/sgml/ref/pg_checksums.sgml    | 16 ++++++++++++++++
src/bin/pg_checksums/pg_checksums.c   | 17 +++++++++++++----
src/bin/pg_checksums/t/002_actions.pl | 10 +++++-----
3 files changed, 34 insertions(+), 9 deletions(-)