Consider switching allow_subdirectory_install filter to a constant

[nacin]

We require plugins to be disabled on Tools > Network. I'd rather a constant get added to wp-config.php than forcing them to modify their theme's functions.php file to achieve the same thing.

I always dropped that plugin in the mu-plugins folder so this hadn't occurred to me. However, leaving this as it is might be a decent way to introduce new network users to the mu-plugins folder.

The most common perception we have run into is that because the mu-plugins folder isn't there, it isn't supported in 3.0.

There's no requirement to run mu-plugins though, especially on a small-scale network, you might as well just network activate them.

I'm going to add a constant, in addition to the filter.

(In [15225]) Add a constant for allow_subdirectory_install as conventional plugins have to be disabled on network.php. fixes #13844.