Add constant for disabling updates

[elusiveunit]

We host a bunch of sites where the code is managed centrally with git and symlinks, making updates a manual task. Having WordPress check for updates is therefore pretty pointless in this case (and in the case of #31011 made our logs get filled with warnings).

Unregistering all the update hooks and filtering some other stuff where needed works, but is a bit of a hassle to keep track of. Having a single constant to completely disable the update system would be great.

The with-30947 patch is based on 30947.2.diff from SergeyBiryukov in #30947 being applied, so it's currently broken for trunk. The other patch does the rest.

I'm not super familiar with the internals, so I've probably missed some areas.

[DrewAPicture]

Hi elusiveunit,

This falls squarely in plugin territory. Core makes some accommodations for managing automatic updates via constants, though I don't predict we would add something to globally disable all updates any time in the near future.

[SergeyBiryukov]

There's a ​DISALLOW_FILE_MODS constant that should effectively disable updates, including core.

[elusiveunit]

I see, too bad.

We do use DISALLOW_FILE_MODS, but the point here was to skip the mentioned update checks that are still unnecessarily run two times a day on dozens of sites. Our hook solution works though, even if it's a bit clunky.