Bug 569665 Enable parsing of compressed files created by OpenJDK >= 15

Improved CRC processing for old Gzip reader.
Tidy up help for acquire dialogs.

Change-Id: I8136f69264c421450fa05409d4a1b92cbde7573e
Task-Url: https://bugs.eclipse.org/bugs/show_bug.cgi?id=569665
8 files changed