Streamline setup-config.php process

[nacin]

As a prerequisite for some stuff in #28577, we need to improve setup-config.php a bit.

When a config file isn't there, wp-load.php prints out an error message then links you off to setup-config.php, which then has an introductory screen. That screen then leads to the form.

What we need to do is insert a language chooser into this process. In order to do that, we need all of WordPress loaded. Here's the plan:

• Combine the error message and the introductory screen. Redirect to setup-config.php the same way we'd redirect to install.php. This will make it much easier to have a language chooser, and it's also simpler. (This two-step dance has always struck me as odd and confusing, and neither page is particularly welcoming in its messaging.)
• Load the WordPress bootstrap as part of setup-config.php. Requires some dancing to avoid DB connection issues. (install.php is fine since the connection is made, even though no tables exist.)
• In #28577, implement a language chooser for setup-config.php, the same that is on install.php.

[nacin]

In 28978:

Simplify the setup-config.php UI flow and load process.

When no configuration file is detected, we now redirect to setup-config.php. This process now uses the WordPress bootstrap, rather than a set of fragile hacks.

fixes #28740.

[nacin]

I would not be terribly surprised if [28977] causes a problem, which is why I spun this into a self-contained ticket and also wanted to get this in quickly. Please reopen with issues.

[SergeyBiryukov]

See #28744.

[nacin]

In 29005:

Don't include plugin.php twice in setup-config.php.

fixes #28744. see #28740, [28978].

[DrewAPicture]

In 29015:

Inline docs syntax fixes following [28978].

See #28740.

[nacin]

Done. Please re-open or create a new ticket for any issues.

[nacin]

This also fixed #12031.