svn4idea / git4idea SSH agent support

- Move the CVS plugin's local fork of Trilead SSH2 into a separate namespace, preventing conflicts with the newer version of Trilead SSH2 used by modern SVNKit.
- Add upstream SVNKit-maintained Trilead SSH2 (patched per http://issues.tmatesoft.com/issue/SVNKIT-178) as a top-level library, used by git4idea and svn4idea.
- Update to current SVNKit, patched per SVNKIT-178.
- Add upstream jsch-agentproxy as a top-level library, used by git4idea and svn4idea.