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

[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

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Commit messages:
 - 8286447
 - 8286447

Changes: https://git.openjdk.java.net/jdk/pull/8659/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8659&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8286447
  Stats: 35 lines in 1 file changed: 30 ins; 0 del; 5 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