<AWT Dev> RFR: 8025673: Disable X11 AWT toolkit

[David DeHaven]

Updated webrev:

http://cr.openjdk.java.net/~ddehaven/8025673/jdk.2/

Summary of changes since last:
- Added awt_headless to java_props_t (set to NULL by default which does not set the property)
- Replaced the toolkit selection code in java_props_macosx.[ch]. I could have just exposed isAquaSession but I wanted to preserve the AWT_TOOLKIT environment variable support (no idea if it's actually used or not...).
- Modified WrappedToolkitTest.sh to check that it's wrapping LWCToolkit in HeadlessToolkit and now all five awt/Toolkit/Headless jtreg tests pass.

No build system or hotspot changes since the last patch.

-DrD-