RFR: 8303530: Add system property for custom JAXP configuration file

[Joe Wang]

Add a system property, jdk.xml.config.file, to return the path to a custom JAXP configuration file. The current configuration file, jaxp.properties, that the JDK supports will become the default configuration file.

CSR: https://bugs.openjdk.org/browse/JDK-8303531

Tests: XML SQE and JCK tests passed.

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

Commit messages:
 - 8303530: Add system property for custom JAXP configuration file

Changes: https://git.openjdk.org/jdk/pull/12985/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12985&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303530
  Stats: 2087 lines in 38 files changed: 1652 ins; 284 del; 151 mod
  Patch: https://git.openjdk.org/jdk/pull/12985.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12985/head:pull/12985

PR: https://git.openjdk.org/jdk/pull/12985