Improve `bst init`, including 2.x upgrade assistance

Currently the story for helping 1.x projects to migrate is not well-defined. Based on IRC discussion bst init could be extended to provide some help for users as follows:

• running bst init where no project.conf exists should create project.conf as usual, but include all default configuration with comments describing what each config does

• running bst init where there is an existing project.conf should attempt to upgrade the project.conf file to match current bst versioning, include any missing configuration, and report any known incompatibilities ideally with guiadance on how to fix