Re-evaluate the decision to make WP_ADMIN => true

There have been a continuous stream of bug reports since #385 landed (particularly b42005e1c3d4ca505a0e507313a4e9f5443df4f7)

### WP_ADMIN: true

WP-CLI breaks completely, because several types of plugins issue redirects and then call `exit`:

* plugins that restrict admin access (#519, #574, #579)
    * workaround is to pass `--user=someadmin`
* plugins that define custom wp-login locations (#582)
    * workaround is to edit the plugin
* the domain mapping plugin (#669)
    * workaround might be to disable its 'admin_init' callback

### WP_ADMIN: false

Plugins that check `is_admin()` before loading part of their code won't work correctly, but most WP-CLI commands continue to work just fine.