[^cdr] IDEA-107195 Methods returning primitive types should be treated as @Nonnull
(cherry picked from commit ca9680c)
[rev=Peter] use declared attribute value in @DelegatesTo's target attribute
svn4idea ssh agent support: Add/update libraries
Support API changes in newer SVNKit release
Implement SSH agent support for svn4idea

- Use modern trilead ssh2 for CVS plugin, removing unsupported
  SSH-over-proxy functionality. (Necessary to avoid conflicts between
  incompatible trilead ssh2 implementations both within classpath).
Consolidate trilead SSH library; add agent support to git4idea