RFR: JDK-8280488: doclint reference checks withstand warning suppression

[Jonathan Gibbons]

Please review a small modification to the way that bad references are reported by DocLint.

A new "mode" is introduced, `strictReferenceChecks`.   

If the mode is _not_ set, references that explicitly include a module name when that module name is not resolved in the module graph will be reported with a (suppressible) warning instead of an error.  All other issues with references will be reported as errors. This is the mode used by javac.

If the mode is set, all issues with references will be reported as errors. This is the mode used by javadoc.

This will need to be documented in the tool guide (man page).

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

Commit messages:
 - JDK-8280488: doclint reference checks withstand warning suppression

Changes: https://git.openjdk.java.net/jdk/pull/7222/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7222&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8280488
  Stats: 311 lines in 5 files changed: 309 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7222.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7222/head:pull/7222

PR: https://git.openjdk.java.net/jdk/pull/7222