RFR: 8296431 - PushbackInputStream should override transferTo

[Markus KARG]

This PR implements JDK-8296431

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Commit messages:
 - Alternative C + Array.copyOfRange
 - Draft: Prevent security issues
 - wrapped long code lines
 - removed unused import
 - Writing available unread bytes in the optimized case
 - PushbackInputStream::transferTo

Changes: https://git.openjdk.org/jdk/pull/10999/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10999&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296431
  Stats: 268 lines in 2 files changed: 262 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/10999.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10999/head:pull/10999

PR: https://git.openjdk.org/jdk/pull/10999