RFR: 8286447: [Linux] AWT should start in Headless mode if headful AWT library not installed [v2]

[Phil Race]

> As per the bug report and the CSR (please review that too), this makes AWT load in headless mode
> if only headless libraries are installed.
> 
> It passes all our normal testing which at least should show that this doesn't regress anything.
> I had to manually remove libawt_xawt.so to verify that it will do as expected in such a case,
> and it would not be easy to create a test for that.
> 
> CSR : https://bugs.openjdk.java.net/browse/JDK-8286598

Phil Race has updated the pull request incrementally with one additional commit since the last revision:

  8286447

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8659/files
  - new: https://git.openjdk.java.net/jdk/pull/8659/files/68b9ec2a..885e1eab

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8659&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8659&range=00-01

  Stats: 2 lines in 1 file changed: 1 ins; 1 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8659.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8659/head:pull/8659

PR: https://git.openjdk.java.net/jdk/pull/8659