Use strict comparison in various files

[SergeyBiryukov]

Trac ticket: https://core.trac.wordpress.org/ticket/60700 https://core.trac.wordpress.org/ticket/58831 https://core.trac.wordpress.org/ticket/57839 https://core.trac.wordpress.org/ticket/56791

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Props sergeybiryukov, aristath, peterwilsoncc.

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[SergeyBiryukov]

This will need tests before it can be merged:

Indeed, this is basically still a draft where I cherry-pick certain changes after a closer review, making sure they are covered by new or existing tests. I will investigate anything related to wp_parse_args() and WP_Query, thanks!