initdb: Change default to using data checksums.

initdb: Change default to using data checksums.

Checksums are now on by default.  They can be disabled by the
previously added option --no-data-checksums.

Author: Greg Sabino Mullane <greg@turnstep.com>
Reviewed-by: Nathan Bossart <nathandbossart@gmail.com>
Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://www.postgresql.org/message-id/flat/CAKAnmmKwiMHik5AHmBEdf5vqzbOBbcwEPHo4-PioWeAbzwcTOQ@mail.gmail.com