RFR: 8273152: Refactor CDS FileMapHeader loading code

[Yumin Qi]

Please review,
  Refactor fundamental CDS FileMapHeader code for reliable reading of basic info from shared archive.
  With the change, it makes it possible to read an archive generated by different version of hotspot. Also it is possible to automatically generate a CDS archive If the archive supplied is not readable or fails to pass the check.

  Tests: tier1-4
   jtreg on sa.

Thanks
Yumin

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Commit messages:
 - Fix comparison between size_t and unsigned int
 - 8273152: Refactor CDS FileMapHeader loading code

Changes: https://git.openjdk.java.net/jdk/pull/5768/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5768&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273152
  Stats: 423 lines in 11 files changed: 263 ins; 51 del; 109 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5768.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5768/head:pull/5768

PR: https://git.openjdk.java.net/jdk/pull/5768