jtreg: ignoring @ignored tests

As many of you know (and are painfully aware!), and for mostly  
historical reasons,
jtreg "processes" tests which have been temporarily disabled with an  
@ignore keyword,
giving an Error result, which messes up an otherwise clean test run.

Since in reality, "temporarily" has turned out to be somwhat longer  
than expected, I went
to fix it. And as I figured the fix, I found that enough of the fix  
was already in place in all
existing versions of jtreg that it can be used by everyone even now.

It turns out that all @ignore-d tests have the "ignore" keyword set  
on them. So you can use
the -keyword option to exclude tests with the ignore keyword by going
	-keyword:!ignore

If you're running jtreg from a shell, you might need to escape the  
'!'. If you're running jtreg
from Ant, you can provide this parameter via an <arg value="...">  
element.

-- Jon G