Offline enabling/disabling of data checksums

Hi,

the attached patch adds offline enabling/disabling of checksums to
pg_verify_checksums. It is based on independent work both Michael
(Paquier) and me did earlier this year and takes changes from both, see
https://github.com/credativ/pg_checksums and
https://github.com/michaelpq/pg_plugins/tree/master/pg_checksums

It adds an (now mandatory) --action parameter that takes either verify,
enable or disable as argument.

This is basically meant as a stop-gap measure in case online activation
of checksums won't make it for v12, but maybe it is independently
useful?

Things I have not done so far:

1. Rename pg_verify_checksums to e.g. pg_checksums as it will no longer
only verify checksums.

2. Rename the scan_* functions (Michael renamed them to operate_file and
operate_directory but I am not sure it is worth it.

3. Once that patch is in, there would be a way to disable checksums so
there'd be a case to also change the initdb default to enabled, but that
required further discussion (and maybe another round of benchmarks).


Michael

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