RFR: 8260265: UTF-8 by Default

[Naoto Sato]

This is an implementation for the `JEP 400: UTF-8 by Default`. The gist of the changes is `Charset.defaultCharset()` returning `UTF-8` and `file.encoding` system property being added in the spec, but another notable modification is in `java.io.PrintStream` where it continues to use the `Console` encoding as the default charset instead of `UTF-8`. Other changes are mostly clarification of the term "default charset" and their links. Corresponding CSR has also been drafted.

JEP 400: https://bugs.openjdk.java.net/browse/JDK-8187041
CSR: https://bugs.openjdk.java.net/browse/JDK-8260266

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Commit messages:
 - 8260265: UTF-8 by Default

Changes: https://git.openjdk.java.net/jdk/pull/4733/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4733&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260265
  Stats: 297 lines in 18 files changed: 184 ins; 9 del; 104 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4733.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4733/head:pull/4733

PR: https://git.openjdk.java.net/jdk/pull/4733