Fix relative paths for renderer commands.

[ehuss]

Custom renderers that explicitly set the command to a relative path was using the wrong base path. On Windows, it was relative to the current directory, and on Unix it was relative to the renderer output directory (see rust-lang/rust#37868 for this unfortunate behavior). Neither are correct, they should be relative to the book root. This changes it so that it is relative to the book root, and has a legacy warning for the old behavior of relative to the output directory. This uses absolute paths to avoid problems with current_dir().

[ehuss]

Note to self: The preprocessor custom command is relative to the current directory, but it should be relative to the book root. Unfortunately the Preprocessor::supports_renderer API does not pass in a context to get the book root, so I don't think it is fixable without a breaking API change. Will file an issue later about this.