RFR: JDK-8181878: javadoc should support/ignore --add-opens

[Jonathan Gibbons]

Please review a simple change to javadoc, to accept and ignore --add-opens.
This brings javadoc into line with the module-related options supported by
java and javac.

The option is simply delegated to javac, for it to handle (ignore) the 
option.
Like javac, the option is undocumented/hidden.

Tests are added to ensure that the option is accepted and does not otherwise
impact javadoc operation.

JBS: https://bugs.openjdk.java.net/browse/JDK-8181878
Webrev: http://cr.openjdk.java.net/~jjg/8181878/webrev.00/

-- Jon