RFR: JDK-8282559: Allow multiple search terms in javadoc search

[Hannes Wallnöfer]

Please review the first of two PRs to enhance JavaDoc search. 

This one focuses on the existing search script, to which it adds the feature to search for multiple terms. It also improves the implementation of case-sensitive search as well as rating of matches and provides some general update and refactoring of the script.

The search section in the help page is updated to mention the multiple search term feature, and I repurposed the last example to show the new feature.

The new search function can be tested here (top-level API doc files only):
http://cr.openjdk.java.net/~hannesw/8282559/api.00/

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Commit messages:
 - Update copyright headers
 - JDK-8282559: Allow multiple search terms in javadoc search

Changes: https://git.openjdk.java.net/jdk/pull/8185/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8185&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8282559
  Stats: 541 lines in 7 files changed: 221 ins; 167 del; 153 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8185.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8185/head:pull/8185

PR: https://git.openjdk.java.net/jdk/pull/8185