Add folder support for the mu-plugins directory (similar to how /plugins works)

[aaronholbrook]

Relating to this ​discussion, and as documented ​here

WordPress only looks for PHP files right inside the mu-plugins directory, and (unlike for normal plugins) not for files in subdirectories. You may want to create a proxy PHP loader file inside the mu-plugins directory:

I propose that we fashion the /mu-plugins to act in a similar fashion to the /plugins folder which looks for plugins either by themselves or in a folder with a named .php file.

-1. I don't see the point. Just use /plugins if you need them to behave the same way as /plugins does.

+1. Some plugins must be mu-plugins, for example when they set cookie constants. There is no reason to restrict those plugins.

Previously: #14359, #5207, #785 and ​http://mu.trac.wordpress.org/ticket/343

Duplicate of #14359 and #5207.