More version bumping to 3.6.

With the creation of the 3.5 branch earlier in the process,
it is necessary to do some of the version bumps now rather than at
final release time (for example, the equivalent of the 3.4->3.5 bumps
in f2bf12fa22c1).  (Some of those changes have already been made, for
example in 30f5e7ec6afe.)