RFR: 8286594: (zipfs) Improvements after JDK-8251329

[Christoph Langer]

This augments the ZipException upon rejecting a Zip File containing entry names with "." or ".." elements.

It furthermore tries to clean up & compact the logic for detecting "." and ".." and it adds a method IndexNode:nameAsString() to return the node name as a String.

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Commit messages:
 - JDK-8286594

Changes: https://git.openjdk.java.net/jdk/pull/8655/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8655&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8286594
  Stats: 73 lines in 1 file changed: 31 ins; 38 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8655.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8655/head:pull/8655

PR: https://git.openjdk.java.net/jdk/pull/8655