8007322: make ftp regression tests optional

[Chris Hegarty]

Given the recent downgrade of the ftp handler by 8000941: "Remove ftp 
from the required list of protocol handlers", the regression tests that 
exercise the ftp protocol handler should be made optional. This will 
make testing the smallest profile and the base module easier ( rather 
than trying to exclude individual tests ).

http://cr.openjdk.java.net/~chegar/8007322/webrev.00/webrev/

Alan,
  I hacked the ftp classes out of a full build to test this. You may 
want to try the patch with a profiles build. All the jdk_net tests 
should pass.

Some notes about the changes:
  - some refactoring along the way to make it easier to
    exclude the ftp parts of some generic URL tests.
  - a few tests use private implementation specific ftp
    classes. Not a problem when using -compilejdk, but
    I needed to add a calling class that exits before
    these types are loaded, when ftp is not available.
  - some code duplication, but I think it is better than
    adding some trivial util helper class

-Chris.