RFR: JDK-8219801: Pages do not have <h1>

[Jonathan Gibbons]

Please review an amended version of the fix for JDK-8215307, which is to 
fix the headings in generated pages to start at <h1>. The previous fix 
failed because of a bug in the test and a dependence on the order of 
iterating the contents of a directory on some systems. The previous fix 
was backed out with JDK-8219558.

After fixing the test to behave consistently on all platforms, two 
additional issues were identified, which was why the test previously 
failed on some platforms. The single- and split- index pages still had 
no <h1>, and neither did the index file written by 
AbstractPackageIndexWriter in some cases.

The fix for the single- and split- index pages was to just add the 
heading, although that did require minor local restructuring of the 
code. The fix for AbstractPackageIndexWriter was to copy the equivalent 
code already written for AbstractModuleIndexWriter.

The fix for the test was simply to reset the current heading "level" 
when beginning to read each new file. (Previously, it was not reset when 
beginning to read the file, and so was left at the last value seen in 
the previous file.) The term is now renamed to "rank" in accordance with 
W3C terminology.

Because this is a big changeset overall, and because the changes since 
the previous version are relatively small, I've attached versions of the 
original patch and the latest patch, with timestamp and similar 
information deleted. This means that an easy way to see the changes 
compared to the previous version is to download the two patch files and 
compare them with a GUI diff tool like "meld".

With these changes, all :tier1 tests pass on all platforms supported by 
the OpenJDK CI system.

-- Jon

JBS: https://bugs.openjdk.java.net/browse/JDK-8219801
Webrev: http://cr.openjdk.java.net/~jjg/8219801/webrev.00/
Original patch, with timestamps, etc deleted: 
http://cr.openjdk.java.net/~jjg/8219801/v1.patch
Latest patch, with timestamps, etc deleted: 
http://cr.openjdk.java.net/~jjg/8219801/v2.patch