deb-upstart: automatically symlink to upstart-job

When --deb-upstart is given, automatically create a symlink to
/var/lib/upstart-job for each given paramter.
Merge pull request #545 from igalic/upstart-symlink

deb-upstart: automatically symlink to upstart-job
- mkdir necessary directories
- style
- give a better error message if the user provides an invalid or
inaccessible path
- give a better error message if you specify a config file path that
doesn't exist.