libappcontroller: Improve plugin search path

In libappcontroller, improve plugin search path by improving 'spath' key value :

• Allow multiple search path using ':' character as separator.
• Allow using environment variables in paths.
• If a path is not absolute, it will be added two times in search paths list, one with binder root directory as prefix, and one with binding parent directory.

Only use 'spath' to compute plugin search paths list, so remove setting search path list using environment variable (still doable using 'spath' key), and therefore, remove using prefix (used only for environment variable search) in controller.