[macosx] Provide means to force the headful mode on OS X when running via ssh

Fixes for JDK-8025673 , JDK-8016096 , and JDK-8003900 dropped support for the AWT_TOOLKIT environment variable because JDK no longer supports multiple toolkits on the same platform. However, the env. variable allowed one to bypass the Aqua Session check and force AWT to start up in the headful mode even if the OS reports that a graphical session is unavailable. This is actually useful when running GUI tests over an ssh connection. Two conditions must be met for this to work properly:

1. A user must be logged in on the local console in a graphical session

2. The ssh connection should be established by the same user that is logged in locally.

If both conditions are true, OS X reports as if the GUI session is unavailable, but still allows an app to display windows etc.

JavaFX relied on this ability to run tests on slave Mac PCs. Now this functionality is broken.

This is request to restore this possibility. Probably we need to introduce a new environment variable, like AWT_FORCEHEADFUL=TRUE or something, that would bypass the Aqua session check, and thus enable the headful mode.