RFR: JDK-8225321 Repeated use of {@systemProperty} in a file causes duplicate ids

[Jonathan Gibbons]

Please review a moderately simple change to have the standard doclet 
better support repeated instances of {@systemProperty} and {@index} in a 
single file.

The problem is that each use for a given name creates a new instance of 
an id for that name, leading to duplicate definitions for the id.  The 
fix is to make the occurrences unique by appending a suffix to the ids 
so that the ids become unique.

Although the bug was encountered with repeated `{@systemProperty}`, the 
fix addresses repeated `{@index}` as well.

-- Jon

JBS: https://bugs.openjdk.java.net/browse/JDK-8225321
Webrev: http://cr.openjdk.java.net/~jjg/8225321/webrev.00/