RFR: 7903775: Add LIBRARY.properties

[Christian Stein]

Initial implementation adding support for `LIBRARY.properties`.

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

Commit messages:
 - Update FAQ document
 - Update tag specification
 - Prevent NPE, take two
 - Only scan libraries for preview flags if the test does not already enable them
 - Prevent NPE for non-package based libraries
 - Build and run tests with `--enable-preview` if they use any library that internally uses preview features
 - Reduce scope of LIBRARY.properties to only `enablePreview=true|false`
 - Decouple library compilation from compile-time environment (read: "enable-preview mode")
 - Draft `LIBRARY.properties` support
 - Make special case of an unnamed library explicit

Changes: https://git.openjdk.org/jtreg/pull/212/files
  Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=212&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903775
  Stats: 146 lines in 8 files changed: 136 ins; 0 del; 10 mod
  Patch: https://git.openjdk.org/jtreg/pull/212.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/212/head:pull/212

PR: https://git.openjdk.org/jtreg/pull/212