Incorporate substantive code from 2.0 stdlib.

This commit only pulls in the parts of 2.0 stdlib that differ
functionality-wise from their 1.9.3 equivalents. When running in
2.0 mode, the 2.0 dir will be searched before the 1.9 dir,
picking up the differing files. We opted to do this to reduce the
amount of duplicate content in our repository.

JRuby 9000 will likely incorporate 2.0 stdlib en masse, and may
only be 2.0 compatible.

Note that 2.0 stdlib is *not* included in jruby-complete or
jruby-jars, since it is still an unsupported mode and we don't
want to bloat up those artifacts any more than necessary.
More tweaks for 2.0 stdlib.

* Revert rbuf_fill logic in net/protocol to using nonblock. We
  fixed SSL nonblock some time ago, which was the reason for the
  modified logic.
* Move our fiddle impl to shared location, since it's custom.
* Remove fiddle from copying in 1.9.3 sync logic.
* Add 2_0_0 to sync_ruby doco.
* Copy versions of stdlib directly from jruby/ruby fork. I have
  created the jruby-ruby_2_0_0 branch to use going forward.
* Update version and revision strings for 2.0 mode.