Exclude 0.9.1-specific patches from devel or HEAD builds
Reformulate HEAD spec to allow devel builds

* Declare all the head build deps as :build dependencies
* Pull the head dependencies out of the head block and make them apply
  to `devel` builds also (assuming that any devel build will be a git
  branch).
* Pull the head spec out of the `head` block
* Add a `build.devel?` guard to the bootstrap build script invocation
  also.